First find out if myrtlewood is ok to laser - some woods are toxic when burned
Then find out where it lies on the Janka scale
Then find the 
material that’s closest to it on the Janka scale
Then run tests going up and down from the setting until you find the one you like 
It works for all materials
In this case:
Seems to be yes
1270
Red Oak at 1220
(though Birch at 1260 is closer and there are lots of settings on here for that)
and test from there