DMAX Function and use

Hi all

can anyone help me with the attached please. I am tying to puu values through from another sheet, as you will see in the formula, but because I have used the MROUND function in the sourec data sheet, I can't seem to get it to work. I thought it would be as simple as some of the things that I have done with other sheets, but this isn't the case.

Anything will be greatly appreciated

Cheers

Parkerlad