Sometimes the difference is negligible so while the rule of thumb is 2 grades. sometimes 1 grade is all that comes off. So you can't make a hard and fast rule on this but you can create a pattern.
Plano, Tx. Card Show #7, Sunday March 5, 2017 from 9 AM to 4 PM at Adat Chaverim (Northeast Corner Independence and Spring Creek) in Plano Tx.