What version ?
I believe the earlier Syil boards can be made to display correctly because they have some pots that can be adjusted, and there is a link that is supposed to change the scaling.
Later X3 pcbs have no pots or link. You are stuck with 50% reading without firmware changes (which don't exist. Any body fixed this?).
Set Mach3 PWM value such that voltage on the PWM signal on the BOB measures 5v at 3600 RPM (commanded from Mach3). That is as good as it will get.
Super X3. 3600rpm. Sheridan 6"x24" Lathe + more. Three ways to fix things: The right way, the other way, and maybe your way, which is possibly a faster wrong way.