The lengths are added to the Recognition Manual in Optical Targeting Correction. The OTC thread can be found
HERE.
As you read the thread, you will understand why the lengths are added to the optical mod fix. It should become clear; Why have correct lengths if the optics are out of proportion? Or better yet, why add the lengths at all?
I should point out that an update to OTC is coming. A version for SH4 patched to only 1.4 is soon to be released.
Also, I have run into a problem with a couple of the "optional" OTC mods that are in the present download. The addition of the "Realistic Scopes" mod will create a CTD when added to the main Optical Targeting Correction mod. I've been waiting to upload the corrected files until the month of May arrives. Just a couple more days.