How to use COUNT functions to subtract 1

How's this?

Formula for O3

=SUM(INDIRECT(ADDRESS(3,2)&":"&ADDRESS(3,2+COUNTIF(B3:M3,"<>"&"")-2)))

Formula for P3

=INDIRECT(ADDRESS(3,2+COUNTIF(B3:M3,"<>"&"")-1))-SUM(INDIRECT(ADDRESS(3,2)&":"&ADDRESS(3,2+COUNTIF(B3:M3,"<>"&"")-2)))/(COUNTIF(B3:M3,"<>"&"")-1)

I probably didn't do the best job at the explaining. In the example, the yellow box would need to be 6 (the total of 30 divided by the previous 5 months) and the blue box would be 6.

Putting in 20 for JUL should give the result of 7 (42 divided by 6 months) and the blue box would be 13. So the Blue looks good testing it out.