sumif but only if category contains values

Hi I'm looking for some help on a sumif formula in the attached file. I have 4 different categories(1 thru 4) of schedules that I want this formula to add together. A lot of different schedules are listed in column A with their corresponding Category number in B. In Column M, a total of needed number of people on a particular schedule is manually entered. What I want just as one example in O5 is if the sum of all category 1's entered totals in column M = 0, to just leave the cell blank. But if they do total a value, I want it to add up all the numbers of category 1 and divide that total by 100. I wasn't sure what type of formula to use in combination with a sumif formula to do this. Thank you for your time.

If i have understood correctly, i would do 2 things
Change the format of O5 to

0.0%;0.0%;0

(this displays zeros as 0 and not as a percentage)

and use the formula

=IF(SUMIFS(M5:M31,B5:B31,1)=0,0,SUMIFS(M5:M31,B5:B31,1)/100)

which first checks if the calculation is zero and if so displays 0, else runs the calc

Thank you. I must have gotten the formula confused then. I thought that you couldn't use an IF formula with a sumif type formula. I'm thinking there is one formula type that you can't use it with but it escapes me at the moment. Thank you for your help!