I want a formula base on cell A1(start month)
this formula should only be under May in E5

I'm guessing that what you mean by
"base on cell A1"
is
"the month of A1 determines which column cannot go over 1."

Maybe it would help to put this in A5 and extend it to L5:
=IF(MONTH($A$1)=COLUMN(), MIN(A3-A4,1), A3-A4)