I've got about a 70 foot +/- run of RG-8X that I use to feed my 40
meter dipole.

Out of curiosity can
someone tell me about how much a a signal loss I've got compared to
having an antenna that's actually cut for 30 meters ?

Hi Gary, According to my calculations the feedline loss is around 90%, so if
you are running 100 watts, and the tuner is lossless, 90 watts are lost in
RG-8X. The good news is 10% efficiency is only 1-2 S units down from 100% on
the recieving end. The bad news is, no tuner is lossless.
You will have loss using a 40m dipole on 30m. Does that satisfy your
curiosity:-)?
73 Gary N4AST