I've read that when using a bike engine in a trike, it's best to use lower gearing than the bike came with to compensate for the extra weight of the trike. But how much lower is too low?

If the bike came with a 3.09:1 final drive, for instance, would a 3.64:1 or 3.89:1 differential be too low? That's a 18% and a 26% difference, respectively. Sounds like a lot, yet there aren't a lot of rear end ratios available in suitably narrow rear ends here in The New World.

id just work it out by reducing the top speed appropriately, ie if the bike did 170 then the trike wants about 30%less, or something along those lines.

bear in mind that the size of the rear wheels can be used to further change the ratio, maybe get some small alloys for the back to reduce speed and increase pull.