Long thread above with lots of correct answers. As the OP, I use an SWM SL3, a PI, to my HR20-100 in my rolling shack. That is dish to receiver with the PI in-line.
This is how I do it:
The PI should be plugged in but the receiver should be disconnected. At least for me that is.
When I park, I use the Android App "Satellite Finder" to get Azimuth, Elevation, and Tilt for DirecTV's Bird parked at Longitude 101 for my exact GPS position. You use can other resources and apps but I like Satellite Finder best.
I pull out my hiking compass and orient myself to the general direction of the stated direction to the 101. Satellite Finder and Dishpointer will also show the Birds through the camera lens. Compass is quick and dirty.
I then plant my TV4RV.com HD Tripod with the included compass in place and included bubble level across the top of the mounting surface. I line its EZ Aim mark to the 101, stomp the legs into the dirt, and level the mounting surface. I pull out my dish, pre-set the Elevation and Tilt, and mount it to the Tripod aligning it with already aimed EZ Aim markings. Most of the time, I am done here. I will have signal. Unfortunately, it's the other times that drive us nuts.
Link to TV4RV.com Tripod:
Heavy-Duty Combo Package
To check, I introduce a DirecTV ASL-1 between the dish and the PI with my Accutrac meter hooked up. It is simply a gizmo that will power the SWM. An unpowered SWM will not generate a signal. If I have a peaked signal, I power up the receiver. If the receiver is powered on during this process, it screws up my meter. If I have 90% plus on the 101, I go get a beer.
Link to ASL-1
ASL-1 Satellite Alignment Signal Locator for SL5-S and SL3-S Dishes (ASL-1P) from Solid Signal
But this is for the other times. Because of magnetic anomalies, aka Deviation (Variation is pretty well charted), something at your local site can throw you off and cause you to lock on the wrong bird. Provided your mast is ABSOLUTELY plumb and you have the Elevation and Tilt correctly set, you can usually slooowly swing the dish either way until your receiver shows the best signal on the "correct" 101. Go get a beer.
For the times I am totally lost, I own a ridiculously expensive gizmo called "Align-a-site". I don't recommend it but it helps, sometimes, especially in the trees.
I settle for 90% on 101 as I use an SL3 with a "Dish 1000.2" reflector. It ain't no match for the giant Slimline but it is much easier to handle. I use an HR20 because it will tune, and record, OTA programming and it is fitted with a 1TB internal drive. Old, yet more capable than the new ones.
Bottom line is:
Know the correct settings for your location. Enter Zipcode into receiver, look it up on the Dispointer.com website, or have an App on your phone.
Pre-set Elevation and Tilt.
Have a accurate aiming system to the "correct" 101.
Have a way to establish an ABSOLUTELY plum mast. NO fudging here (unless you have a properly calibrated Align-a-Site). An askew mast will mess with everything else.