If you can plot the taper you can figure it out mathematically with a fair degree of accuracy.
Consider also that the ferrule is of a heavier mtrl than the wood and would therefore require additional calculations.
Just how accurate do you want to get with this???
Seriously, your shaft should still weigh 4oz +.
Don't forget, you're taking it off the end that's already been made smaller. There's not much left to lose.
It could come down to how heavily the sealer is laid on.
KJ