I've been slowing down. Let me know when to roll in when you guys agree.
The variable that challenges the whole process in the flight levels when doing science is the changing groundspeed during the turn. The often dramatically changing radius of turn frequently calls for proper tongue placement and eye squint in order to rollout on the desired track. Automation? No workee. Be a pilot and use the turn knob, with an eye on the MFD wind vectors (if available).
I'm sure there is a formula, but I don't lug around an enormous brain. Keep it light; keep it simple (thank you old-school bush pilot Oren Hudson) works for me. Having a headwind on the base leg for the turn on helps, too.
