Newbie Questions about my mega farm ebb&flow system

[DT4]

Hi,

I am new to hydroponics and decided to buy a Mega Garden system by HydraFarm to get started on this learning adventure. I have a 4 tube T5 fixture inches above and have a couple of questions:

How deep do I place the 1.5 inch rockwool square? Will the hydroton draw any of the water upward beyond the level which the water floods? If I placed the rockwool so that the bottom makes contact with the very top of the water, it seems like it would be buried quite deep when planting something like lettuce.

Thanks in advance.