1500fpm is still 3 degrees albeit at a higher speed.
Fly the approach at 450kts.
You're overthinking this one.
You probably have a chart for 60 or 65% power cruise. That's pretty close to the power setting you'll be using during the descent with flaps out and gear down.
Without knowing the scope of the assignment you're doing and the detail of the charts you have I'd suggest 15 min at 65% for the approach.
In real life you take a best estimate and always round up.