Yet another blowout question

[BigMapes]

Hey folks! I've been researching quite a bit and can't quite find a solid answer. I have a Watts 1" RPV (I believe). For the longest time, me and my landlord have blown out the Sprinklers via the port on the left (I use 60-80 PSI from my 12 gallon, not a huge system so I only need to really run it once per zone, 5 zones, and the sprinklers blow out then drop down within 1 minute or so). I did it this year as well but after some research, it almost sounds like if you blow out using these test cocks, you should use the one on the right (I've since moved the chuck). Ideally, it sounds like there needs to be a blowout port installed further downstream but this backflow preventer is under the house and my landlord keeps telling me he'll move it outside one day since it's a PITA to get under there. Am I tracking right here? Thanks in advance!