Excel Formula in VBA code

I'm currently getting my VB userform to look at a certain cells contents then just display based on whatever that box is. But If i move those boxes etc it crashes out on the userform and I have to go change location of where to lookup etc.

Is their a way I can to get it to read in the 3 values I set then do the calculation fromVB rather than do the calculation on the excel form then display thats output?

Thanks in Advance

As in if I move the content to a different cell location, e.g. may be reading from a2, then I move and it would read from g55 for example

attached an example of what I mean, I just want to do the working out between two dates for the particular drop down, I know the calculation, just messing up atm, because in the userform i change the dates, drop-down option then it changes the excel contents then excel changes the contents based on what ive just worked out to 15 for example then displays it in a box which i display on the userform

I just basically want it to work it all out in the vba code based on my cell contents