A key depth is 2ft above bottom w/o weeds for every 5ft of water and 1-2ft above weeds if they are present. But, this ranges and you may have to play around a bit. You are allowed 3 lines, so you can vary each tip-up and see which depth produces the most action and then counter the other 2 lines to that depth.
On L.Poygan last cpl years when the Pike were big and hungary, I did my very best 1ft under the ice. Was this because they were roaming just under the ice or was it because they were used to dead smelt being up that high, I have no idea. But before I knew my depth, I played with 6 different depths (had buddy w/me) and it payed off.
Hope this helps and gives you some kind of an idea on where to start. It's all a learning experience as you will learn something new everytime you are out.
Tightlines,
Gatorbait
