Dont over complicate it if you dont have to. If you can hunt the field you're seeing them in, why not just go there instead of worrying about the roost?
It's more important to know where they are going once they get off the roost than to know where they roost in most cases anyway. If you already know that, you've got more than half the battle won anyway. Get to a spot at or in-between where they are going and set up. If they fly down and change their pattern, hopefully you hear them and have an idea where they are going and you can adjust your plan.