Casting my mind back I ran 8.24:1 on a 13x2 in my Pred and 8.6:1 I think in the Mako.
Working on that and the ZX5 having a 2.5:1 internal ratio and 78 tooth spur that would put you in 23t region.
Now bear in mind that was with 3300s (back in the day) and BRM motors which as Glyn often reminded us needed "gear tit oopp" so ended up about 3 teeth higher than something like a Reedy of the same wind. Based on that I'd suggest starting on around a 20t or 21t pinion and seeing how the motor is performing.
So long as it's punchy and not too hot after say 2.5 minutes of running I'd continue for the full 5 mins in practice and then look at it's brushes / com .... if they are still the right colour your not far off.
If it's more than warm after 2.5 minutes then I'd stop and change gearing, that is how I used to do it. If it was sluggish and getting warm at 2.5 mins then I'd go down 2 teeth ..... if it was crazy punchy and hot I'd go up 1 tooth ... repeat till you find the best gearing for that motor ...... BEWARE .... IF YOU GET IT TOO FAR OUT YOU'LL COOK THE BRUSHES OR WORSE!!!
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