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Spacing between the reflector and driven element should be
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Gain vs Spacing
The gain of an an antenna with parasitic elements varies with the spacing and tuning of the elements, and thus for any given spacing there is a tuning
condition that will give maximum gain for this spacing. The maximum front to back ratio seldom, if ever, occurs at the same condition that gives maximum forward gain.
The impedance of the driven element also varies with the tuning and spacing, and thus the antenna system must be tuned to its final condition before the match between
the coax and the antenna can be completed.
However, the tuning and matching may interlock to some extent, and it is usually necessary to run through the adjustments several times to insure that the best
possible tuning has been obtained.
(ARRL Radio handbook)
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