Ok I went and did a bit more looking and checked again with the Zeta and some 12mm MDF I found a scrap of. It actually measures exactly 12mm, not 12.7 as is most common here.
Using the 4mm offset plate it punches through considerably as seen in the first pic.
In the 2nd and 3rd pics I added an additional 2mm thick piece of perspex to give a total offset of 6mm, with a tiny blowout on the back of the 12mm.
So it turns out the timber I'd used wasn't 12mm thick, but varied between 13mm and 15m depending on the project, with 13mm being the minimum sweet spot to get the connector high enough on the mitre without actually blowing out the back using a 6mm offset plate. I don't think 12.7mm was quite adequately thick enough to not get blowout but can't remember properly for sure.
If I can find some 13mm I'd do a sample test to illustrate this.
When you say you use it for drawer boxes, what exactly is this? If they are being lined on the inside, you could actually make a 7mm thick offset plate that would blow through very slightly on the inside where the solenoid kicks in with the grooves cut by the blade, but it should leave the other side intact without any blowout on 12mm thick material.
