Calculating Percentages Of Multi-level BOM

So our company gets Material Statements which basically state what chemicals and how much of said chemicals go into producing X item.

I need to figure out the percentage of MASS for each chemical.

So data might look like this:

Total Weight of product: 500 grams or mg, etc.

Sub-Item - Contact - 450 g
Material - Copper - 425 g
Substance - Nickel - .05 g
Substance - Copper - 422 g
Substance - Lead - .02 g
Material - PVC Insul - 5 g
Substance - Polysomethings - .5 g
Substance - Red Dye - .4 g
Substance - Plastic - 4.1 g

So basically the sub-item line is 450/500
first material line is 425/450
substance is .05/425

the substance lines use vlookup to find the material line to divide by. formula i'm using is below:

=IF(B2="Sub-Item",I2/$M$2,IF(B2="Material",I2/VLOOKUP("Sub-Item",$B$2:I2,8,FALSE),IF(B2="Substance",I2/VLOOKUP("Material",$B$2:I2,8,FALSE),"n/A")))

B2 is either sub-item/material/substance
Col I is the mass of that line
M2 is the total weight of the item

Formula works until you get to the Polysomething lines in the above example data. instead of taking the material weight from PVC Insul, it takes it from Copper.