Key for me is a history of days-past-date paid (date of check, days-past-date of invoice(s) paid, and $ amount).
 
What I do now is to put the days-past-date in the AR-C Check No. field (I can always get that from the paper stub if I need it -- never have), like "32"; if it's a range over several invoices, I put in the low and the high, like  
"3246".  I come up with that before, by reference to the Aging, so I have it for AR-C.
Then I scan the "check No." AR-R to see how someone's paying.
Average would be good, but only if you can set the date range and show $'s.
 
The other datum I need to do some poking around for to come up with is a Credit Reference Request for a customer's "Recent High Credit", but that's lower priority and doesn't look too easy to do.
 
Thanks for asking.
 

Which prompts the next question - define "Average days to pay" - Average since the beginning of time or only the past X months to reflect current conditions?  And should it be somehow weighted by $ amount so big invoices carry more weight?  
 
Current calculation for the AR-A value takes the current value averaged with the payment entered regardless of the $ amount so 50% weight goes to the most recent payment, 25% next prior, 12.5% next prior, etc so it is extremely skewed to the most revent payment(s)