Heidi, min 32x mag to achieve rangefinding. bigger mag in general better. a large objective lens will also help. sidewheel easier to use. you are trying to get a shallow depth of field. ie in/out of focus in small movement of s/wheel or front lens whichever type of focusing your scope has.
big nikko good for reasonable price, widely used so plenty secondhand.
i'm not perfect, but I shoot FT, reality is this is pretty much the same thing!