All reticle-rangefinding is based on the most basic form of the mil-dot mil-ranging formula, from simple plex to Ballistic plex--it's all the same equation just various units. I always use my multi-stadia reticles for rangefinding at the optic's highest power.

Here's the equation that i use--

tgt. size (") x range of reticle subtension measurement (usually 100 yds.) / subtension (") / quantity of "gap" tgt. occupies (decimal equivalent) = range (yds.)

Here's a couple pics to show how it works for a mil-dot that's calibrated for a power lower then the highest (12), and then mil-ranged at the highest power (18) where the mil-dot subtension is now 2.38 inch per hundred yds.--

tgt. size-12", range = 300 yds.

1st pic) 12 x 100 / 3.6 / 1.15 (maybe?) = 290 yds.

2nd pic) 1200/2.38/1.7 (edge of tgt. is below x-hair slightly) = 296