I really cannot understand the reliance on mathematical calculations and geometry. There will be a circuit height which you should attain by downwind leg and a circuit speed or suitable speed for your particular aircraft. At some point on the approach the descent starts and from then on the only dial you should be interested in is the ASI. Everything else is Mk 1 Eyeball. I know that this is a simplistic version but who needs instruments to fly a basic trainer? The aircraft will tell you if the speed is right and the positioning is up to you.
I am not an instructor, I used to be an Assistant Category Gliding Instructor back in the `60`s but no longer.
To the guy who first posed the question:- Don`t worry, (be happy), we all have some bit of learning to fly which gives us a problem, In my case it was controlling the direction of the glider during the roll after landing. As many people have said, it will click one day. There`s no hurry, the guy in the other seat will be there for a fair while yet. If you REALLY have a problem, lessons every day for a few days will probably sort it out but don`t try too much each day. The mind seems to need to absorb and sort out the new information during sleep.
Regards,
Mike W.