Ahoy hoy,
I use a 70 qt extreme; and my preheating is a bit different then others. I fill my entire cooler to just under the lid indent, with the hottest water my hot water heater puts out, then put on the lid, and leave it till its time. I do this right before I start to grind grains, which is when i start heating my strike water. Since i do 2qts per pound, i usually have a strike amount of 7-10 gallons, which takes an hour to an hour and a half to get to about 170 or so (this is all indoors on my stove, outdoors under propane 1/4 heating time) so by the time its ready the coolers been preheating close to 2 hours. i then drain it, and refill it with the strike water. I also take into account ambient temps and stuff, and compensate as I feel the need, but Ive always found its easier to cool the mash once mixed, then to heat it, so better to have heat overkill, then under kill. It works great for me that way. Your milage may vary.
FYI
A great day to you all.