I have never heard that before, don't worry about "ratios" between GPU core clock and GPU memory clock. It's possible that you heard something about the core to shader clock ratio, but that doesn't apply here.
I would just continue pushing the core clock as high as possible, and then do the same for memory. Do one at a time, leaving the other at stock value so you can determinetthe true max speed of each. Increase voltage as necessary but don't exceed either the max voltage or max temp. Benchmark as you go to see the gains for each bump. Sometimes the increase in speed can actually have a detrimental effect on performance for some reason. It's good to know ahead of time where these peaks and valleys are, as well as knowing how much benefit there is to each component's overclocking.
Once you have figured out each max clock, then you can clock them both up together. Continue benching to see the gains, and to ensure your GPU's running at optimal performance.
The 560Ti is still a pretty decent card, I'm actually on the lookout for a good deal on a 560Ti 448cores 1.5GB card for use in a Folding@Home rig as well as for dedicated PhysX processing. A friend has one that I may be getting for $125 (with a full waterblock), and I've gotten his up to 1098core and 2280mem with only a small voltage increase.
The difference between stock and overclocked is significant, as he's running 2 in SLI (3770K delidded with PK1 @ 5Ghz with an XSPC Raystorm Copper, Maximus V Extreme with MIPS Full Board Block, 2x4GB G.Skill Trident X DDR3-2666 10-12-12 @ 2738 10-12-11 1T, 2x EVGA GTX560Ti 448cores in SLI with full coverage blocks @ 1098core/2280mem, Plextor M5P-Xtreme 256GB, 3x WD Black 1TB HDD in RAID0 @ 418MBs, and 2x HWLabs Black Ice GTX 480 rads using Push-Pull 120x38mm Koolance 2600rpm fans wi, 1x Alphacool NexXxoS Monsta 360 rad using Push-Pull 120x38mm Koolance 2600rpm fans wi, MCP35X2 pumps, Bitspower 400mL Res, Swiftech Maelstrom Dual MCP35X Bay Res, BP fittings and 1/2x3/4" PrimoFlex Advanced tubing, Phobya 120x20mm Black Shrouds on Push sides of all 3 rads, XSPC radiator gaskets between the radiator/fans/shrouds and fans/shrouds, 2x Lamptron FC-Touch fan controllers for the Case Fans (8x 140mm + 7x 120mm) and rad fans (22x 120x38mm Koolance 2600rpm fans), a custom 9.6"-diagonal 1680x900 Color LCD touchscreen with a desktop stand and a single cable leading back to the case plugged into a custom PCB control board with 12x Temp Sensor inputs + 8x Flow Sensor Inputs + 10x LED power plugs + 12x 4pin Fan Headers with RPM reading + 4x connectors to allow for expansion boards (he's an Electrical Engineer with a second Masters of Computer Science & Engineering) with 6x Aquacomputer In-Line Temp Sensors, 6x Air Temp Sensors (all sensors calibrated) for ambient/rad-in/rad-out, 5x Aquacomputer In-Line Flow Sensors that can show either the avg or list individual values, 6x 2" long 8-LED RGB-LED strips with control over color (3x in a 3/8x1/2" acrylic tube running in the middle of the BP res, 1x connected to the Swiftech res, and 2x in Lund Plexi Res LED things; with the Mayhems Pearl White dye and the LEDs on bluish white, the entire reservoirs glow), and all the "auxiliary" fans connected and controlled (MCP35X2-HS 80x15mm Delta 4800rpm fan, 3x HDD coolers custom made w 2x 80x15mm Noiseblocker fans wired together each, G.Skill RAM cooler with fans swapped for some 60x20mm 5500rpm ones and linked together, and some misc 80/92/120/140 fans) as well as the pumps speeds via rewired connector w split Tach wires so each still goes to the MB but also this neat display/controller. The case is a custom job, similar to the CaseLabs MH10 in design but the interior is different. Has 3x 140mm fans vertically on the front left and front right, 4x 120/140 fans on top on both sides, 2x 120mm MB side (left) exhaust fans and 2x 120mm PSU side (right) exhausts, 3x 4-drive cages on the right, 3x 5.25 bays on either side at the top above the fans/HDD cages and a pair of slot-load slim ODD mounts facing up along the top midline, 2x PSU mounts, removable motherboard tray with 20mm standoffs and 12x PCI slots that are adjustable to fit standoffs between 3mm and 35mm long, large real-glass Window, constructed of 2.6mm thick aluminum sheet and 5mm thick L-brackets,a sesecond fan intake panel between the drive cages and motherboard that is 1.2mm aluminum and has 3x 140mm fans vertically aligned blowing air across the motherboard, the MB tray has a huge backplate cutout around 4x4" and mounts for 120/140/150/180mm fans with a Corsair SP120PE mounted blowing onto the rear of the CPU socket, one of the Lian Li 2x120 fan brackets mounted with the fans blowing straight onto the motherboard and GPU's, and a single Lian Li 1x120/140 and two 2x120/140 aluminum PCI fans having the single one between the GPU's and the duals on either side of them blowing cold air onto the PCBs (actually dropped temps a further 4C, from 31C to 27C under load).
Sorry for the wall of text, I just love the case! Plus, I have been bugging him for months to build me one of those custom remotely located touchscreen whole-PC Controllers (it shows all water and air temps and even gives the current Delta-T down to the hundredth of a degree, avg or all flow rate sensor readings, both pump speeds down to the RPM, the speed of all connected fans, the temps from all onboard sensors (CPU Package and cores, etc), and even shows the CPU BCLK + multiplier + RAM speed + GPU1 & GPU2 Core/Mem clocks, and all voltages! You can touch one readout area and it pulls up a log of that sensor/tach/etc and stores the data on a 128GB USB 3.0 flash drive or 128GB SDXC UHS-1 card, polling sensors every 0.5-5sec depending on the sensor, and keeping up to 300 days of completely synced logs before you have to change the flash/SD drive!).
Wow, that was way more than you needed. Sorry! Hope you found the help, helpful! And the rest, well, interesting