I think the OP has the cooler method down, he's asking about larger brews in a 20 gallon Blichman. I have 40 gallon kettles and chose to insulate. It's cheap and efficient. I lose about 1 degree per hour. I've thought about going RIMS because sometimes I have trouble hitting my mash temp right on the button, but I've been getting a better handle on my heat loss here lately and dialing it in. You'll lose heat from any pumping you're doing and also from the mash tun if it's not insulated. If you brew indoors, and the ambient temp is about the same everytime you brew, you can get a handle on your heat loss by doing dry runs with water only. Figure out what your average stirke water temp will be. Heat that and transfer to an empty MLT. Take a temp reading and how ever many degrees short you are, add that to your strike water temp in the future. Then, do an infusion and use a value of zero for your grain amount in the infusion equation. Stabilize the temp and take a reading. If you come up low, adjust the value of the infusion water in the equation until you find the number that gives you the temp that you ended up with. For instance I lose 11 degrees during transfer. I pump 200F water but I use a value of 189F in my infusion equation.