I would like to put the Bowen Rough Country rear sight (or an equivalent) on my 686+ with 3" bbl. Does anyone know the correct way to measure for these sights?

Bowens web sight has a chart with model numbers and the sight for it. If you model is not listed send them an email and they will it you know which sight you need.

Thanks. I emailed them earlier today and their response arrived a few minutes ago. I was trying to measure the height of the front sight but they need the rear sight height.

Unless you have a "need" for the rough country sight, I would recommend their target sight. That rough country sight is a PITA to adjust compared to the target sight - however, if you only have one load for your revolver and you shoot mainly at one distance and hold-off for other yardages, then it's other a great sight.