The following is just my own opinion and thoughts...based on my own experience. I'd advise that you do as much research and experimentation to figure this out for yourself. I might be completely wrong...I quite often am (or so my wife tells me
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There's no such thing as "timing". But as long as you've installed the sector gear/tappet fin as described you're fine.
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This is only applicable to semi-auto shots...for full auto you will always run the chance of stopping the piston anywhere in it's cycle.
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What's happening is that after the cutoff lever knocks the switch block out of contact (i.e. it cuts power to the motor), your motor/gear combination is still spinning (e.g. freewheeling under momentum), and pulls your piston back for the "next" shot.
This is good/not-good. The good part is that your shot time for the next shot is shorter because the piston will not have to be pulled back all the way, sort of like pre-cocking the rifle.
The bad part is that if you store it like that you're spring will loose a good portion of it's oomph and you'll get very low FPS. If the spring gets so weak that the gears start another cycle before the piston is all the way forward...you'll either lock up the mechbox or strip the gears/piston.
To prevent this you can install a MOSFET unit that has active braking. AB sends a quick reverse pulse to the motor to freeze the gear train very shortly after the power is cut.
Or you can try a stiffer spring. By installing a stiffer spring, there will be more pressure pushing on the piston which will resist the over spin of the gears/motor when the power is cut.
You can also slightly overshim one of your gears to create "drag". The tight gear will act as a brake and kill off the over spin. The down side to do this is that your motor will have to work harder when trying to drive the gear train when pulling the spring, so you'll notice your useful battery life is greatly reduced.
Hope this helps, best of luck.
Tys