Thanks. So with the G MX518 I should set "850" DPI to get the same setup as with my old mouse at "800" DPI?
You need to measure this yourself. Every mouse can be a bit different.
I mean, use a ruler and measure something in real life on your mouse pad. For example, I like to measure how much you need to move the mouse to reach from the left edge to the right edge on the screen. On a 2560 pixel wide desktop with 800dpi, that's 3.2 inch, and 3.2 inch are 8.1 cm. On a 1920 pixel wide desktop, it's 2.4 inch or 6.1 cm.