Main vent location

I went to a job other day, (steam) had a wet loop that leaked , I fixed that but the main vents (2) where on the returns but at the very end on the tee , it wasn’t raised or 18 inches from the end. The only good thing is that it’s bushed on top so I can make a tree of main vents. I’m looking for picture or a diagram of options of how to but some main vents.
If I get the job I’m planing on taking the main off and see how long to get steam to that point. Then try to get a few mains to vent fast.