Jools – can't see direct drive working. I started out by using car installations as a guide, where starter to engine ratio is around 12:1. As my rollers are 4” dia and a rear wheel is approx 25” OD (giving c. 6:1 redn) I figured I’d only need 2:1 or thereabouts between starter and rollers - forgetting, of course, that on a motorcycle the engine turns faster than the rear wheel, therefore making a nonsense of my guesswork calculations. Hence the later addition of a layshaft to introduce a further reduction of around 7:1. Roughly speaking, the roller / tyre reduction is cancelled out by the wheel / engine overdrive. In which case I’d suggest your bike starter / roller reduction should be similar to the starter / engine reduction found on a bike.
In conclusion, I appreciate my setup is a tad agricultural but hey, I wanted light, cheap and portable.