Terry: As you know, I shoot 45 pounds most of the time.
If I am going to use 400 spine arrows, I need to load up the total front-end weight to 325-350 gr or more for a 29.5 to 30.5 inch shaft. (this is a combo I have been experimenting with recently)
For 500 spine, I will load the front end to about 225-235 for a 29.5 inch shaft. (this is my regular hunting arrow)
A lot depends on shaft length and what I am looking of as arrow weight once everything is put together.
In general, A 400 is better for 50 pounds and above, while 500 is better for 50 pounds and below -- given the ability to adjust the front weight with insert weight plus point weight until the arrow shoots well.