This is my preferred test method:
If you are masking and you’re still seeing excessive burning on the back, go back to step 1 and test:
you’re overpowering the cut.
When cutting extremely fine points you are almost always going to be prone to some charring. To combat that follow @geek2nurse’s advice, but if you really want to get the best results in these cases try doing multiple fast passes. Pick a fast speed (300+) and then back into the number of passes you need to make to get through the cut at about 85 power. The combination of slightly lower power and higher speeds will make the least impact to your surrounding wood. You can get very precise delicate cuts using a method like that.
“So, evansd2 you do that all the time then, right?” Nope, I almost never do it. I go hot and fast, a slight bit of discoloration is either something that I will accept with the design or I will modify the path to eliminate thin spots. My time is the most important thing to me, in general I will change my design before I’ll make the job take longer. YMMV.