just as w5lz & airplane say....MEASURE MEASURE MEASURE and remember, the tuning coil is the final 'tweak'. put the tuning ring, wire & bar where it should roughly be & adjust the length of the antenna to get your best swr. adjust the tuning ring & wire 'in the air' for 1.1:1 EVERY TIME! mine has nearly 1.7 mhz bandwidth UNDER 1.5:1! 27.3=1.1:1, 29=<1.7:1, 25.6=<1.7:1. oddly, 29.6=1.5 & 24.950=3:1.......the mfj259b answers the 'why' for that.
to tune most ANY antenna 'in the air'-take a LARGE truck tire, monuted on a rim, make a 't' from 2x4's & bolt through the lug holes. experiment with YOUR best way to mount a pice of pvc tube, or galvanized pipe to the 2x4 't', mount your antenna on 10' or more of mast & put it on the 'tire-mount'. a beam may need to be 'guyed', the v5/8 never has on my 22" truck tire test mount. a wooden extension ladder does not skew the swr for 20-10 meters-though a 20 meter yagi needs a bunch of guying! if you can get an antenna such as the v5/8's feedpoint 12' off the ground, it usually doesn't change once on the roof. even if only 6' above the roof!