Don't understand your table logic! You table will be have some logic when:
1. You need a formula in column 'WITHIN 90' returning "Y" when something described in table row is paid within 90 days from 'BEGDATE', "N" when it was paid later than 90 days from 'BEGDATE' or is not paid and currently more than 90 days is passed from 'BEGDATE', and returning an empty string when it is not paid and currently less than 90 days is passed from 'BEGDATE'. When this is the case, you need an additional field in your table (e.g. 'PayDate'), where date the payment was finished will be stored;
2. You enter into column 'WITHIN 90' manually "Y" or "N" when the payment is made. When this is the case, you need the information, is the entry within 90 days or not, to use right character. One possibility is to have an additional column with formula which displays "N" whenever the entry is out of 90 day limit and payment is not made yet (there is no entry in column 'WITHIN 90'). Or you use conditional formatting in your table to color entries out of 90 day limit and payment not made in one color, entries paid later than 90 days in another color, and entries paid earlier in 3rd color (no need for additional column then).

The formula for additional column in 2. will be something like