Help fix my IF formula or recommend a better formula method please!

I have a travel claim that may not always have value's in cells I basicly want this formula to =N56-M16 if greater than 0 and M15 is 0 OR =N56-M15 if greater than 0 and M16 is 0 or N56-M16-M16 if all greater than 0 and if M15 & M16 =0 then =IF(N56>0,N56,""). I know that last one works by it self but this was too complicated for me to figure out and this is what I tried. =IF(M15=0,0,(N56-M16)),IF(M16=0,0,(N56-M15)), IF(M15=0,0,M16=0,0,N56)
I have considered putting the possible equation outcomes in another cell but still don't know how I can explain in the fomula how to reference them. I have uploaded the file because I for one find it easier! :o) thank you!

PS I think I can get my same results with these cells If N30=0,N4, If N30<0, N4-M16 because N4 has the amount due but includes the total from N30 which on the account summary I have to have a separate line item. But I still am not sure where the () go.