I'm more used to what would I would call regular 'dirt' soil. I use a moisture meter to tell me when to water. I recently started using this bonsai supply soil. I love the way it drains and looks. I want to transition my other bonsai to it in the future but its so coarse my moisture meter doesn't work on it. Outside of using my brain and thinking about when I watered last like I obviously need to do, is there any good way to determine the moisture level of this rocky soil?

I just look on the surface. If it is dry on the surface I water them. For beginners it may be easiest to use akadama, moler clay or something similar on top that changes color quite much between wet and dry.

In any case. I don't know one bonsai artist that use moisture meter and noone that count days between waterings.

Ohh, yes. I know people that prefere to lift the tree and judge need of watering by the weight of the tree and pot.

If you are into measuring devices it would be possible to use a scale. Proboably more reliable than those moisture meters. One just need to buy two and compare the measurement to understand how reliable they are.