Trial and error was my method when I raced my Rotax. Maybe I was too cheap to buy a jetting program, but I just tried out different jet combinations to see what worked for me. Provided you don’t go too lean (I had a 150 in my tool box, but rarely used it. Like Joe said, 152 is about as low as you’ll go in CO) it’s tough to hurt a Rotax. The beauty of Rmax rules (provided that’s what you’re running) is that the Jetting is pretty restrictive, so there aren’t that many combinations to try. When i was first starting out, I would just ask other people what they were running for jets/needle and tune from there…gave me a decent baseline.
My advice would be to play with it. When you’re out test & tuning, change something different on the carb each session and try to watch what it does. Every engine is a little different, so not everyone’s setup will work on your engine (my second Rotax liked being about a jet size leaner than my first). Listen for high pitched ‘popping’ when you’re WOT on the straights; if you hear that, you are (likely) too lean…bump the main up a size.
Your mileage may vary, but that’s what worked for me.