when i use 68 fov in game but in the config it says 90 should i use 68 or 90 in game and il play 4:3 1024 768 do i have to multiply the sens by 12.5 to get same turn speed??

If you are talking about CSGO, 68 is probably your viewmodel_fov, which is irrelevant when it comes to sensitivity. Only the fov_cs_debug value matters, and only if you play on a server with cheats enabled. Otherwise it will be locked to 90.

so when I do calculations use 90 fov