If one of the multiple cells is blank, then show a return value to only a specific cell.

Hello, I am working on a project about load computations for my Reinforced Concrete Design Subject.

It is based on ASCE/NSCP load combinations. What I need to do is whenever I input values(loads) on i3:i8, it needs to compute the value based on which formula is suitable, otherwise, if the cell is blank, do not use the formula.  For example, if I input a value in D (which is i3), the formula that will be used is only 409.1 since the rest of the values are not given, and the output will be shown on cell C19.  So if I input values in D(i3), E(i8), and L(i4), and the rest are blank, it will only use the load combinations from 409.5 with the output shown in L19, since it is the only suitable combination for it. Also the other load combinations, if not suitable, should be blank, or must return a value of "Do not use".

So far, the formula I used is =IF(ISBLANK(I4),1.4*(I3),IF(SUM(I4:I8)=0,1.4*(I3),"Do Not Use")) . That is for the load combination 409.1, and the output must show in C19.

The first problem is that since I only have a value on D (i3),  it should only compute using a load computation from 409.1, but excel also computed using other combinations (since all of the load computations contain D, that is why E19, H19, J19, and L19 have values which should be blank. )
The second problem is the return value should only be in a single cell C19, but here, it is also returned on C20:C23.

The last problem is when I input a value in R(i5), the return value in C19 must be Do not use. But mine returned a value from the original formula. 

Sorry for the grammatical errors.