I plan on brewing the Chainbreaker IPA recipe from Zymurgy next weekend. It calls for a step mash: 15 minutes at 125, 15 minutes at 145, 20 minutes at 163 and 5 minutes at 172. BeerSmith assumes I am going to heat the mash tun directly, but since I mash in a cooler, well, you know. . .

I found a couple of calculators online that tell me to add 19.11 quarts at ~134 to hit my first step, the 6.6 quarts for the second, 10.6 for the third and 8.9 for the last. That's a total of 45 quarts of water added to the mash for a 6 gallon batch. That sounds a bit off to me.

The way I see it, I have two choices: First, divide that 19.11 into 4 additions (which I don't know how to do); Second say Phaqit and just do a single infusion mash with 19.11 quarts.

Anyone have any advice?

For a 3 step infusion mash, you want to dough in thicker, around 1 qt per lb, and have it recalc for the right strike temp.
Have plenty of extra boiling water and cold handy so you can adjust the steps. The trick is to calculate and measure accurately, and overstate the thermal mass a bit. Then, add the water slowly so you come up to temp gradually. You can always add more step water (or decoct), but you can't remove water. You only have so much cooler space.
Also, be prepared to do an emergency decoction if you run out of cooler space and you miss your temp low.
Basically, get a pan from the kitchen as well as the pot you used to use back when you did extract. Use the pan to scoop up a bunch of grist, and be sure to decant off most of the loose water so it's thick like oatmeal. Fire up the stove, and put the thick grist into the old boil pot and bring it up to 150 for about 10 minutes, then raise it to a boil, stirring constantly so you don't scorch.
Return it to the main mash to raise the temp some more. Be sure to scoop more than you think you'll need. The biggest problem is you tend to not scoop enough and you STILL end up low.

HTH-

There is an option for muti step infusions in beersmith. It's down near the bottom in the recipe design or on the mash tab.

I used that. It reads "Protein Rest: Add 19.11 gallons of water at 132.7. Sac Rest: Heat to 145 over 15 minutes. Mash Out: heat to 172 over 15 minutes." I mash in a cooler. Putting the cooler on the burner is problematic, hence my dilemma.

That's what beersmith calls a 'Temperature Mash', but what you want is one of the 'Double Infusion' mash profiles. You can customize one of the standards beyond light, medium, or full body. That will give you the chance to change your infusion volumes and temperatures to hit your rest temps.

Still a good idea to keep some boiling water and ice around to tweak it if you miss and you should still add your infusion water slowly to creep up on things. Good luck.

"Well there's your problem." -Mythbusters

Thanks! Didn't even see those.

This still doesn't make any sense. Add 13.76 qts of water first, then add 12.23 quarts for the next step, and 10.70 quarts to mash out. That's 36.69 quarts total. That's a shade over 9 gallons of water for a 6 gallon batch. I'm gonna have to boil the bejesus out of this!

Screw that. I'll just do a single infusion mash and only use 19 quarts of water.

Yea, I skipped step mashes until I had a way to add heat (direct fire for me). Take the easy way out. Or crank down your grist ratio on the protein rest to 1 qt/lb and see if that's reasonable.