Mash Percentages when Combining Raw Distillates

[OneFootKev]

Hello ADI,

I have searched but haven't come across a straight answer to this question - I'm sure there are many versions of the process I will describe so I will do my best to lay all the info out. 

The aged products we currently distill include bourbon, empire rye, corn whiskey and apple brandy - all with different mash bills. My question is concerning mash percentages of those products...

For an Example: If I were to add 50 gallons of bourbon feints to a corn whiskey mash (or another combination of distillates) prior to distillation, how would I go about determining mash percentage? These two products contain the same ingredients, just in different ratios - where if I were to add some rye feints I would be introducing an entirely new grain into the mix.

Thank you all for your contemplation