And even more flex watt questions!

I have a few more questions re. Flex watt...

For those of you that use the plainer to recess a groove into the shelf when putting the heat tape below the containers, how deep of a groove do you make? What I am wondering is does the flexwatt actually have to be in contact with the bottom of the container or will it be just fine to have it slightly below? Will the heat rise easily enough?

Another question is that if on the very top of my rack I put a strip of flex watt, can I use aquariums on top? They wouldn't be getting dragged over the tape or anything on a regular basis, but I wonder if the black rim on the tank which holds the glass up off the shelf will allow for enough heat...

I never did get many people's oppinion of whether the 3" strips of flex would be enough or if I should go with 11". The shelves will be about 24" from front to back. Each shelf will have one container 37"x22"x6 1/2" then another that is 10"x20"x6 1/2" next to it. The top shelf may have a number of the smaller tubs side by side...

Thanks guys!
Rebecca