If you want a formula, how about this?
39 tonnes = 3.9˚
For one tonne difference adjust descent by +/- 0.1˚
For every 0.1 Mach less then .78, reduce descent by 0.1˚
For every 15 kts head/tailwind adjust descent by +/- 0.1˚
Allow 0.3˚ for Mum and the kids.
Enter the resulting value in the descent angle perf. page. The number is generally about 3.4˚until of course you get a big tailwind... Then watch the bugger; it will try and overspeed you given half a chance.