And even then it doesn't mean you will be able to tune every frequency between 3.3 and 30 Mhz.....
However, if aiming for the HAM bands (7 Mhz, 14 Mhz, 18 Mhz, 21 Mhz, 24.9 Mhz, 28.5 Mhz) I found out that a 9.2 m wire (this is isolated backstay portion or sloping wire PLUS the lead from it to the ATU) is a good length:
close to 1/4 wave on 40m
enough under 20m 1/2 wavelength
and for 15m it is about 0.625 WL or the "hald enf-de Zepp" length with maximal gain lobe and still low-take-off angle for DX.
Above 15m it starts to be a cloud warmer.
If I had to cut and isolate my backstay and for a given antenna tuner, I would:
- loosen the backstay and run it close to the mast
- hang a sloping wire from a spare halyard
in the same position as the backstay, to find a good compromise wire length to reproduce aftwerwards with the backstay
(even if the wire diameter, copper wire vs stainless, etc will give lsightly different results)
...or install the SGC-230 it tunes nearly everything....