Robb,
Here is a link to a DIY page on my website that does detail heat setup on racks.
http://baileyreptiles.com/cb70rack.html
I place my probe directly on the heat tape, however, some others do it differently. The key point to probe placement is to know the temperature in the tub on top of the heat source. I have my Helixs set to 98 degrees (probe on the heat tape), which inturn gives me a temperature of about 93 degrees inside of my tub. Just make sure that you don't have to turn your thermostat up to high (say 110 degrees) to get 92-94 in the tank, or you may risk melting the tub.