I tend to work to the rule that if there are more than 5 levels of nesting required, there's a better way to do it.
There are more fancy ways to achieve this, but on the basis that each group of four original and conversion grade/size values is unique, you could do it with a simple SUMIFS formula.
See picture below.
Columns A:E represent your conversion table as shown in the picture. Arrange them in one table in rows down the page.
Columns G:K (G3:K3) represent your inputs, so you provide the grade/conversion pairs and a quantity.
The formula in L3 then gets the unique 'Factor' and multiplies by the quantity in K3.
Formula in L3 is =SUMIFS($E$3:$E$6,$A$3:$A$6,G3,$B$3:$B$6,H3,$C$3:$C$6,I3,$D$3:$D$6,J3)*K3
If I've understood your requirements correctly VLOOKUP won't help you in this case, as it cannot do multiple criteria.