There are several different FOV types to consider. R6 uses vertical degrees, and the calculator automatically switches to the correct type for the game. If you want the same FOV as in CSGO, you switch the type to "Hdeg 4:3" (which is what CSGO uses) and enter 90 as it is in CSGO. The output will tell you the config FOV for R6 to get this FOV:
This get people confused quite a lot, 68 is the viewmodel FOV, not the actual first person FOV. fov_cs_debug is the correct variable for the FOV.