How would I treat the decimal in the hexadecimal number 4.BF85?
Would I write 11*16^-1 + 15*16^-2 + 8*16^-3 + 5*16^-4 ?
You could do that.
Then you could work out the numbers. $\displaystyle 16^{-1} = 0.0625$, and so on.
Then (after lots and lots of tedious calculation) you would get your number in decimal format.
Strictly speaking, what you're calling the "decimal in the hexadecimal fraction" is a misnomer, it's only a "decimal" in the base ten system. Its technical term is "the fractional part".