Laterite quantity, substrate circulation & reptile under-tank he

Regarding the use of laterite, how does one calculate a 
quantity to use?  I think I saw this information in FAQs, but when
I needed it I couldn't locate the information.

I'm considering the same substrate set-up approach as Granger Ridout, 
that is, a base of vermiculite, laterite & fine gravel mixture and fine gravel 
on top...awaiting comments to his post. 

 I have one additional idea I 
would like to solicit comments on.  Since circulation is important in the
substrate, I was thinking of taking half-inch CPVC with numerous 
holes drilled in it and running it 
throughout the substrate to facilitate circulation.  I was going to 
place it on top of the vermiculite with a clear tube coming up from the 
network in the back of the tank to add nutrients and fertilizers.   A 
couple of air stones 
could be placed inside to aid the movement of water when CO2 is 
turned off.  

To further 
treat the substrate, I was thinking of using the "Heat Wave" reptile undertank 
heaters by Hagen.  There are three sizes up to 25 watts and they are 
reasonably priced.  Any comments?