I use a lot of cheap Baltic Pine for functional boxes and such which aren’t meant for display. Today I discovered I trick that completely eliminated scorching on the wood (well, this time). All I did was apply two layers of medium-tack transfer tape to the underside of the wood instead of one. One layer on top as usual (since I was engraving too).
Maybe I got lucky this time (likely), but the results were really good!
If your backside is overcooking you are probably hitting it too hard in the first place. That balance of “enough power to be reliably cut” and “not so much that you torch your opposite side” is a little tricky to find.
I might try to dial it in a bit more but if you’re set on using more sacrificial material, give this a shot: 1 layer masking but place a piece of cardstock down first, underneath your ply. The cardstock should eat up the excess laser energy and the one layer of masking ought to protect the wood from smoke/etc.
Fair! I use the Medium Maple Plywood setting because it is almost right and I’m too lazy to fine tune it 
That’s a pretty low bar you’ve got there 
Fine tuning is a quick process if you use my method…
You can get zeroed in very quickly, ten mins tops, including setup in the UI.
Once you setup the ui, testing subsequent materials is a matter of a minute or two.
I’ve used the cardstock trick to shield the huge token jobs I’ve done that required pulling the mask to save weeding thousands of pieces.
It has to be faster and cheaper to get the settings right than to apply multiple layers of masking every time you cut!
While I appreciate the thought, I think that might not be true.
Early on I spent a couple hours trying scores of settings getting them just right for my cheap wood, only to have them not work well on the next piece of wood from the same batch. At 62, my time is very precious
Double masking is fast (like a minute tops) and nearly free. I do like the card stock idea though, that’s even faster!
No matter what your age, your time is precious. It just takes most of us far too long to realize it.
i still think your settings are just running too hot.
fwiw, i run standard maple ply settings with BB and single layers of masking and i have no issues with flashback on the back side. i’ve got some custom settings i’ve used occasionally when the sheets were slightly thicker than .125" instead of slightly thinner. and it’s always important to note that .125" is nominal thickness and i’ve had BB anywhere from .117 to .132. and that difference is enough that just using the maple ply setting doesn’t always cut through with the thicker ones.
is it possible you’re not getting a good adhesion with your masking?
sometimes if the sheet is either a little dusty or the grain is really pulled up (so quality dependent on the surface), you can wipe down the sheet first or even give a super quick buff with a random orbit sander before putting the masking on and it holds better.
the other trick is to make sure you’re using something to “burnish” the masking onto the wood. even something as simple as the edge of an old credit card is all you really need. if it starts to pull away even a little, you can get flashback on the back or burn under the masking on the front.
I can relate. I’ve dialed in settings on one end of a board only to have them fail on the other end. Same grain, no knots, tight against the crumb tray. Natural variation is natural variation. The cheaper the materials the more likely you’ll get settings that were over-powered yesterday to be under-powered today.