I know if you’re charging a 12s4p pack with p42a cells at 8a, that’s charging each cell at 2a. Is that basically dividing the charging amps by the p group?
Yes, you got that right. 
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I think it is wise to have an inline meter on one’s charger output.
It can help one see issues with their battery, or charger, or the circuit, before they turn into something worse than the walk of shame.
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You’ve got it right.
Keep in mind when cells get their advertised amperage rating (charging or discharging) it is in the context of a single cell in open air.
So assume a pack will heat up quicker and try to stay well below the advertised limits for any long term current draw.
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To expand on what @jack.luis said: all cell ratings should be assumed to be one cell in open air. Charge and discharge rates are also for new cells so the rates have to be assumed to decline over the life of the pack. As the cells age and the ir increases and total ah declines the pack will heat more during charge and discharge cycles and you should plan for that in how you rate a battery build.
When im doing insane charge and discharge batteries i build for 70% outputs and inputs AFTER derating the cell for being packed in like sardines. Then i monitor the first and worst charge and discharge to make sure i didn’t forget something or get hot spots. If i can reasonably charge a new pack without hitting thermal cutoffs when new i need to have enough of a safety margin that an aged cell can do the same. Just because i could do 35a charging on a new pack doesnt mean i can do the same on a pack halfway through its lifecycle if it was approaching the thermal cut at the end of the cycle 
always monitor charging 
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It’s a good idea when designing most things to only plan on using 50% of THINGNAME_RATING
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Ok cool, i got a new 16s4p pack with p50b cells and for a 10a charger. Looks to be charging at 2.5 amps per cell. What ball park charge rate per cells seems excessive?
The p50b’s will laugh at 2.5 amps charging current, even in a pack inside of a dead air enclosure.
I don’t think any other cell currently available can in theory be charged as quickly as those.
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Sweet! Thanks!