And welcome to Shooting the breeze.
Most people in FT use a high mag scope on the highest mag to judge the range. This is acheived by marking the parallax ring/wheel with the ranges. (example - when the 20mtr target is in perfect focus, the wheel is marked 20mtr and when the scope is focussed on the 30mtr target, the wheel is marked 30mtr)
So when you have marked all the ranges on the wheel you can then focus on any target and the mark on the parallax ring/wheel will indicate the correct distance. You can then work out your aimpoint/dialling.
The higher the mag, the better this works.
Hope this helps.
All the best