Everything depends on your situation and aircraft type.
Is there any rough formula I can use to determine speed for max angle?
As a rough rule of thumb:
300kts = 30 000'/min (over the ground obviously)
250kts = 25 000'/min
etc.
What is your aircraft's ROD's for these speeds with spoiler and/or gear/flap?
Disregarding the level bit at the bottom where you have to slow down from the high speed descent, then 3000'/min @ 300 kts will give you a 1/10 descent, where you only need 2500'/min @250 to get the same angle profile (or 2000'/min @ 200). Anything above these values for the speeds given will give you a better angle.
I have found to my disgust that slow and dirty beats fast and steep almost every time.