need to adjust stock by something other than 1 unit based on attribute purchased

When someone purchases an item of a particular size in my client's store, I need an option to adjust the stock by the value of the attribute instead of -1?

Any ideas? Anyone want to take this on?

Backstory...

I would like to manage an item's stock level based on which attribute is purchased and not how many (although I would expect the quantity field to still be effective). I can imagine this might be of use to anyone who stores their inventory in bulk but sells it in sizes that are measured just prior to delivery.

I've got an inventory of chemicals stored in bulk volume (e.g. 127mg). In my store, customers choose to purchase 0.5mg, 1mg, 5mg, 10mg, or 25mg. (no, nothing illegal)

We can't divide the inventory into units because we don't know what sizes will be needed and don't want to disturb the inventory unless absolutely necessary.

Customers would think we were ridiculous if we asked them to purchase 50 x 0.5mg units when they wanted 25mg of something. It's just not done.

So, we it set up so that customers select an "amount" attribute which lets them select how much of the stuff they need and get the appropriate price.

The problem is that the inventory management is still setup to remove one unit from stock when we need it to remove 0.5, 1, 5, 10, or 25 "units" from stock as people purchase in different "sizes".

So, what I need is for the inventory management to look at the attribute value (0.5,1,5,10,25) and remove that number of units from inventory when an item with that particular attribute is purchased.

(BTW, if it's too much trouble to track fractional units, we CAN track inventory as a quantity of 0.5 units. (e.g. 10mg = 20 0.5mg units)

I would imagine this would be of value to just about anyone who deals with "pour-to-order" situations...