Custom Pricing: TAX percentage detraction

I added TAX percentage detraction to the Custom Pricing module calculation types.

This is a common issue when you have a source pricelist with final price and you want to offer to a specific class of customers (typically resellers) a tax excluded price. In this case you cant just decrease the price by a percentage for them

ie: reseller price = original price - (original price * decreasing percentage /100)

ie: tax 22%, original price: 100 

reseller price = 100 - (100 * 22/100) -> 78

but if you try to re-apply tax: 78 + 78 * 20 /100 = 93.6 ... it should be 100

this is not the right way. You should instead subtract the tax percentage from the original price:

reseller price = 100 / (1 + 22/100) -> 83.33

and if you re-apply tax: 83.33 + 83.33 * 20/100 = 100