I have the formula to calculate feedline length for *monoband* V/UHF
antennas, and was hoping that somebody could shed some light on the dual
band dilemma either through calculations or personal experience or both

Light has already been shed. The length of the feedline needs no formula.
If you insist on using a formula, try the following:

Measure the distance between radio and antenna, double it and then divide by
two.

"Best Performance", particularly on VHF and UHF usually means reducing
feedline losses. This is done in several ways:

1. Use the best quality, largest diameter coax you can afford.
2. Use the shortest length necessary.
3. Make the antenna resonant and impedance matched to your coax.

This last point may be the source of your misunderstanding. When the
antenna presents a resonant match to your feedline (1 to 1 SWR) the current
and voltage along the feedline don't vary; they are the same regardless of
where you measure them. Therefore it won't matter if the feedline is a
multiple of 1/2 wavelength...current and voltage are the same along the
feedline.

IF HOWEVER, your antenna is not well matched to the feedline, you will have
varying amounts of voltage and current along the feedline. The SWR will be
greater than 1 to 1. Under this condition, you can trim the feedline length
to improve the "apparent" match. This may allow your transmitter to put
full output power into the transmission line, and improve performance. But
the feedline will have increased losses because of the increased SWR. The
correct fix for this condition is not to trim the feedline to a particular
length, but to adjust the antenna for a better match.

73,

Roger K6XQ