Hi there! I've never bred BPs, but I've successfully bred my leopard geckos, and I guess this sort of thing would carry over. Don't quote me on that, though

You really don't want the probe to be outside of the egg box since you are measuring the temp where the eggs are gonna be.

Here's what I did:

I had enough space in the incubator that I was able to create what I call a "dummy" box. In it I recreate the exact same environment the eggs will be in, but there are no eggs in it. I drill a hole in the top of the egg box container just big enough to stick the thermometer probe in, and that way it gets the air temp inside the box.

So in short, measure the air temp in the environment the eggs are gonna be in. You can't accurately get the temps of egg surface during incubation anyway since youd need to open the egg box and doing so would let out the heat you are trying to measure.

Like I said, I only bred leos so this may or may not work, but I don't see any difference here that would keep it from working.