The calculator is accurate so long as you've given it accurate input. Provided you have done so, and are at the correct viewing distance, the sim world is to real-world scale, so you have accurate sight-lines etc.
The main criticisms levied against this are:
- "Everything looks too wide"
Perception issue, if you want a less accurate view then by all means increase your FOV.
- "I can't see my dials"
This is just the result of the overall aspect ratio of your setup. There is a good argument for having, say, 3 28" 4:3 monitors instead of 3 24" 16:9 monitors to give you more vertical viewing area to get around this. iRacing have implemented an adjustable VanishY parameter to try to help with this, bound to shift+[ and shift+] by default (might be ctrl, unable to check atm)
- "I can't see my mirrors or anything particularly to the sides"
Single monitor user most likely; in which case see if you can get yourself closer to your screen first; failing that you'll have to increase the FOV to as low as you think you can feel comfortable with in terms of peripheral vision. This is the reason why triple screen setups are so sought after these days, consider investing in one! Alternatively, consider a TrackIR.