[sjbird]

leaves 80 for the vehicle and initial starter-battery charging. Does this sound reasonable?

It means, for a battery 100AH, it will charge a fully discharged (50AH, not dead) in 40 minutes.
Say if promaster has a parasitic draw of 0.1A (hopefully less), and was left for a week, it has lost 20AH
Plus starting an engine @ 600A for 15 seconds = 3AH (hm, I did not expect it to be so little)
It will take 15 minutes to charge to back to full.
80A is 1.3AH/minute.
Absolutely reasonable