Trial and error. You’ll need to enter the ingame settings, then “Advanced Settings” -> “Manual Correction”, tick “Enabled”, and play with the correction values. In case of Diablo 2 (where I’m just using my Diable 2 layout), it’s easy: 640×480 (game resolution). In Windows 95, you’ll also have to enable “Block mouse move” and use “msmouse” correction in most cases.