Calculate negative values

Hi All,

I have a problem with a formula I am using to calculate reserve storage.

I have tried using an advanced formula but it is not producing the result I need. Rather, my formula is displaying negative values in some cases (see attached reference file highlighted cell E13).

If the formula is going to produce a negative value, I want it to display a blank cell instead.

I have attached a working file for reference.

Can someone point me in the right direction PLEASE?

Thanks