In my very limited experience raising one I found that keeping the substrate just deep enough for them to dig a hole their body depth or a little less is a good compromise. As long the temps, RH, feeding, and the substrate is nice and damp (not wet) this is enough substrate in my opinion. Then you get to see your frog!

As far as how wet the substrate should be you should be able to grab a handful and squeeze just a few drops of water. Less is to dry, more is too wet...