Whenever you use a combination of transmission lines like that, it's good to
calculate the impedances and SWR at various points in the system. That's
because the wrong combination can cause very high losses.
For example, say you have a 90 ohm impedance at the antenna (which is
a ballpark value for 20m.) If the twinlead is 1/4 wave long, it would
transform the impedance up to 1000 ohms at the coax junction, so the
SWR on the 75 ohm feedline would be 13 : 1. But if the twinlead was
1/2 wavelength (as it it is with the G5RV feed system) then the impedance
at that point would be 90 ohms, for an SWR of 1.2 : 1.
When you consider the range of impedances that the tuner might encounter
at the end of a random length of coax, it could be between 6 and 1000 ohms.
Of course the feedpoint impedance varies with frequency, as does the
electrical length of both feedlines. So you have to do this analysis on each band
that you want to use, like this:http://vk1od.net/antenna/G5RV/index.htm
If your interests are primarily 80, 40, 20 and 12m, then using the standard 20m
half wavelength of twinlead is a good approach. If you operate more on 17m,
15m and 10m then putting the balun on the table beside the tuner in the shack
is a better choice, so the coax length is a matter of inches.