What's wrong with 3 times your height and 5 times your ground speed plus a bit of slowing down room? And keep recalculating it as you descend.
Failing that use FPA, put the line where you want it and manage your energy correctly to meet any speed restrictions.
Lets face it, you can plan all you like but ATC will invariably muck up your planning anyway (no disrespect to ATC but that's the way it is), slow down, speed up, maintain your level, descend early or late etc..
Anyway, how do you know you are a third of the way through your line training?