That thermostat is proportional so you'll be good using that. I love my 300x2!

As far as the probe. I put my probe in an extra egg tub, with perl and water. Sometimes I put it in an egg tub with eggs (if I don't have any extra egg tubs ready). I have also put the probe in the middle of the incubator for an entire season. In my experience it doesn't matter what you do, it will all work out as long as you don't have any spikes.