I have a total cell amount that can go up to any amount. I also have a Grand total cell that uses the total cell amount BUT it needs to be a total number that is < = to 20

I have a TOTAL cell amount that can go up to any amount. I also have a GRAND total cell that uses the total cell amount BUT it needs to be a total number that is <= to 20.  So in other words. It the TOTAL = 31  the GRAND total can only = 20.   If the TOTAL = 8 the GRAND total would equal 8.

Thank you!