im really struggling to get a formlua to work

what i want is

i have 2 varibles which i need to compare and if the figure is less than 70% tell me how many is need to make the 2nd figure 70%

eg A=12 and B= 5 so C= 41.6%

i have this so far

=(ROUNDUP((B17*H3)-C17,0))

B17= a user defined varible

H3 = 70%

C17= is a user defined varible

In this example B needs to be 70% of A

the formula i have works and tells me how many i need to make 70%

ie. if A=26 and B=15 then the answer is 4

what i want is a formula so that takes into account A and B will both increase by what ever amount the result of the formula is, so i need this to be taken into account

any ideas