Re: Ffp or second focal plane
You can also range with an SFP by correlating the reticle subtension with the known size of an object.
The old Leatherwood system used (and still uses) SFP to bracket 18 inches if you're using meters, or 16 inches if you're using yards, to auto-range from 200 to 600. Their newer models might be something other than the original 3-9 (3:1 ratio), so maybe they now could to 200 to 800?
If you don't have a coupled auto-ranging setup, then you just read the power ring and do some basic math. Trust me, with practice you can get quick at it.