I have an older house XM tuner that has three separate radios built into one stereo deck with a PRO-600 antenna. I had trouble getting good reception the first time I set one up, but when I got the new antenna I learned my lesson.
I used one of the "Satellite Pointer" apps most FTA hobbyists use to help me get the general direction right. I believe I chose XM 4 (115W) and Sirius XM 5 (85.2W) which give you a good range. That helps you figure out where obstacles might be (trees, etc). I've got one of the old portable XM radios (the one with the battery and headset) so I took it outside during the setup and used it to help me fine tune the signal. You can still find those on eBay cheap, or use your regular radio with an extension cord, etc if that is an option.