sum using or (if Blank cell in Column)

My Basic problem seems to be that i cannot run a variable against a blank cell.
My set up as below:

Item 50cl 70cl 1.5cl per 25ml ( answer i'm looking for)
Rum £17.50
Cheese £80.35
Milk £10.50

Now individually i can work out these items. Rum would be 0.62 in the 25ml
column... but I cannot find how to create each sum in the total box, so when
i leave two of them blank it understand to only work out the one answer.

It must look something like this? Right?

Item 50cl 70cl 1.5cl per 25ml
Rum £17.50 =sum(b2/20) (C2/28) (D2/60)
Cheese £80.35 =sum(b3/20) (C3/28)
(D3/60)
Milk £10.50 =sum(b4/20)
(C4/28) (D4/60)


What am i missing in between each set of brackets to make the thing work
though?