Really Dumb Herms Question..

I'm planning on converting to Herms.....and planning to buy another kettle to convert into the herms HLT. I have a 10 gallon boil kettle, and a ten gallon MLT....do I get a ten gallon HLT or 15 Gallon ??? To My line of thinking (since I only do 5 to 7 gallon batchs) If I use ( In a ten gallon pot)up to 8 gallons of strike and sparge water, will there be enough left in the HLT to cover the heat exchange coils for recirc heating of the MLT. Or am I missing something?

Just finished my herms setup and what I do is fill both my BK and HLT with water and heat them simultaneously. Then I use my BK as my mash water, I transfer to my MLT. Then you have a full HLT to keep the mash temp consistent, and use as sparge water.

I would get the 15 gallon and use that as your BK. That way you can go larger in your batches if you every want too.