Problem is with decimal separator.
If I use "." all is ok(15.7 →15.7). If I use "," only int part saved (15,7 → 15).
During custom reticles editing all numbers showed with "," like separator.

As result we have:
1.Edit reticle.
2.Input 15.7.
3."Ok"
4.Edit reticle.
5.See 15,7.
6."Ok" (size of reticle on FOV window change)
7.Edit reticle.
8.See 15.

Fixed.
Bonus: The confusing dialog-within-a-dialog has been replaced by an all-in-one window. Any changes made are reflected instantly in the FoV chart.