Reference what I mentioned above that the ECU doesn't know the physical position of the IAC. If you pulled the IAC out and operated it, you probably noticed that once its pulled in all the way, it still steps but it doesn't move. There's some type of clutch in it to prevent damage. So, knowing this, the only way to the ECU to have repeatable steps to your desired position is to always start at one of the extremes, either all the way in or all the way out. That's why it says to set the homing steps larger than your maximum steps. You can choose the homing direction under the idle control tab. If you have homing direction set to closed, you have your polarity correct for that coil. In my opinion, you want homing direction closed. I have my homing steps set at 300. This guarantees it's closed all the way when the key is turned on. After that, it will go to the setpoint you have in the idle cranking duty/steps curve. There's plenty of time for it to make both of those moves if you wait until the fuel pump finishes priming.
I would install the IAC, make it move 250+ steps to home (closed), then unplug it and take it out to make sure it is in a position that appears closed.
After this, you can build a curve using the test mode by dialing up steps while the engine is running and up to temp. Record the amount of steps and RPM. Then you can make your warmup curve from there. Oh, you should also see the plunger move in (open) when using the test mode for run. If not, one of the coils is backwards.
On mine, I would have thought it would want 0 steps when the engine is full warm, but it doesn't. I'm around 35 steps at 200 degrees. I'm assuming that's from the aluminum expanding and closing up the tolerances.
I hope I explained this well. Honestly, the IAC is the biggest pain to get right with megasquirt. I've probably spent more time messing with that than I have anything else on it. Maybe I'm a dumbass, but like you said, their instructions aren't very intuitive.