Acceptable is settling for less than perfect. If it's got errors, you'll either need to back down clocks or bump up voltage. Don't pass 1.1v on air however. When you reach top clocks without errors and stable system is your GPU OC limit. All GPU's OC different limits and luck of the draw.
My default voltage is 1.0872v. I found out that default voltage can vary from GPU to GPU. I was always wondering why other people would get lower temps than me at stock core default voltage. I currently lowered mine to 1.0750v which brought down my temps by 1C and still run stable.
Here are some of my 580 SC clocks to help you.
The GTX 580 SC is 3% gain higher than a GTX 580 vanilla at stock speeds.
39-40C idle @ stock
797Mhz core on SC which is a 3% gain over vanilla stock.
40-41C idle @ 838Mhz core on SC which is 8% gain over vanilla stock. 838 Core/1061 Memory/1676 Shader
41-42C idle @ 855Mhz core on SC which is 10% gain over vanilla stock. 855/1086/1710
42-43C idle @ 875Mhz core on SC which is 13% gain over vanilla stock. 875/1111/1750
43-44C idle @ 890Mhz core on SC which is 15% gain over vanilla stock. 890/1135/1780
To go higher stable I have to bump my GPU to 1.1000 voltage.
44-46C idle @912Mhz core on SC which is 17% gain over vanilla stock.
Now when I OC I bump up across the board for Core / Memory / Shader to be exactly the same percentage gain on each GPU-Z validation.
I got clocked to on my final score 3D Mark 11 mid level performance score of 6902. On an i7 950 @ 3.8Ghz OC - GPU 1.1000 voltage @ 912Mhz core.
3D Mark 11 score of 6902
Good luck with your over clocking, Mahalo.