any way to recreate this formula in a saved search

Haven't tried it but if it is possible, it doesn't seem like it is without hard coding the time frame into the search formula itself since you're looking at all AR from the beginning of time over sales in a specific period (I assume).

a formula field like this might be enough: FLOOR(NVL({paymentdate},{today})-{invoicedate}) make the summary type of the field = average; show totals for the saved search.