Equipment profiles in BeerSmith are just a launching point. Same with the Mash profiles. You'll modify both to fit your exact system and brewing methods.
Start with the Kegs profile.
Change the mash tun size to 20 gal.
Change the brew size to 15.5 gallons
Change the loss to trub to whatever you measure
I'd suggest just doing a "water batch" to start. Simply fill the HLT and run the system through it's paces. You can do this with or without heat, but using heat will help you find the rise times and the boiloff rate.
With new systems, I like to do a basic beer as the first batch. Light Ale, Pale Ale or Wheat beer, none of which go too far out in gravity or hopping rate. Something I've done before and know what the wort should taste like post-boil. This always gives me a guideline for how efficiently the system will work, and I get a risk-free, decent everyday beer out of it.
Once that batch is chilled and measured, the resulting "Measured Efficiency" number can be used in the equipment profile for Brewhouse Efficiency. From there, it's just small tweaks.
