I've added side plates of offcuts to keep the heat away from the back when heat treating Yew, I found this helped to spread the heat along the limb so that rather than doing an inch or so at a time ( doin up the brown dots) it gave a better spread of heat and gave about 45 mins per limb.
There's no substitute for taking its slow.
I still remeber with embarassment when I first tried it and posted results "showing it didn't work" nothing wrong with results... they clearly showed that rushing it and just charring the outer wood acheives nothing.
I was slightly miffed that I din't get much helpful input other than 'get TBB vol 4' and read it. But hey, it does the job. Bottom line is if it takes less than 45 mins per limb you are going too quick .
Best aid to consisten results is a kitchen timer.
I forgot a limb once and it toasted right through
Del