what are the kind of calculations that need to be done in your head?
I'm at 5500' and I'm thinking about landing. The airport's at about 500', so Traffic Pattern Altitude will be 1500' (500 + 1000)
That means I have to lose 4000' (5500 - 1500)
If I want to do this at 500fpm, then means it'll take me 8 minutes (4000 / 500)
If I'm travelling at 180 knots, that means I'll travel 24 miles (8 * (180 / 60))
And I want to get there say five miles out, so I need to start the letdown at 29 (24 + 5) miles out.
In fairness, this only LOOKS complicated. You'll be doing the same calculation again and again and it'll become (5500 - 1500) * 6 plus a little bit, and when you think about it, you only need to be as accurate as you can fly.
And then there's fuel burn too, and weight and balance.
Good luck
Actually the calculation will really become "Oh look there's that house. Already? I'd beter start descending"