sighting

Traveling up the A71 today and being a nosey b I called in at Morton Clay Targets for the first time. Well under construction is a 100m indoor rifle range (now at the fit out stage). Speaking to the owner it will cater for 8 positions, all segregated and due for opening end of July. For anyone...

Any tips on range estimating. I have just been told try one eye closed with a finger on target. Close the eye then with the other there will be a gap, estimate the gap measurement between both positions and multiply by 8 (or 10) and that will be the range. I've yet to try, anything similar out...