With a limited amount of scrap/test material with which to work, consider to keep your settings on the low power/faster speed for engraving. If you set things too low/too fast, you can easily increase it until you are satisfied.
For example, I will crank out a test engraving at 200 mm/sec and 20% power for new material. If it’s too weak/too light, it’s simple enough to push the power a bit higher, if the speed isn’t causing alignment/overrun problems. If there’s overrun at the higher speed, slowing down can help in that respect.
It’s a bit of a crap shoot to determine how much higher to go if the first shot is too weak. Is it really really weak? Crank up the power doubled to 40%. Is it nearly deep/dark enough? Maybe 30% will be sufficient.
If you’re moving from thin card/easy cutting and rock hard slate/high power stuff, consider that wood is wood. You could find some other scrap that isn’t as critical and see what the power settings can do for a block of pine scrap or similar.
Birch plywood is pretty soft, pine is pretty soft, you’ll get close results with both and get a better starting point for your critical pieces. I’ve been seeing pine scraps on a regular basis, they aren’t hard to find. Sand down a scrap to get a smooth clean surface and fire phasers.