Don't feel bad, I was somewhat of a self-taught jumper until I started lessons some twelve years ago or something... I STILL wasn't taught distance until about a year ago. So needless to say I had a very honest mare that tended to pick the distance for me.
You want to find your distance, as you described it. The strides it takes to get there. You "estimate" and adjust your horses stride (in the canter) whether it be to half-halt to shorten the stride or push to lengthen it. I'm still pretty bad with Fiona on finding my distance because she hasn't learned yet to really lengthen her stride. She will shorten it, so more often than not I will try and slow her stride prior to the jump, which tends to work... But just avoiding the inevitable :P
All it really is is practice, practice, practice. Take a ground pole or two (on each side of the ring/pasture/whatever) and try and count your strides up to it. Adjust in half-halt or squeeze to lengthen as needed. It IS frustrating, so try not to fluster yourself. If you get frustrated, take a break.
Or you may be a natural, unlike me :P
The Fluffy Equestrian
Rhett | Hang on "Fiona" | Rockstar
Reg. TWH Gelding | Irish Draught Horse Mare | SSH Gelding