When you know at what yardage your gun and ammo is
patterning right and you want to keep it within that range,

Stick your arm out at 90*, stick up your thumb, close your
non-shooting eye and see how much of the target your
thumb nail covers the target or where the target is in relation
to your thumb nail. Remember your yardage and mental
sighting.It's not exact, but close.
Use in the woods to calculate that yardage on turkey's.
I see you want to shoot as soon you see the bird, so I would use
#5 or #4 for the killing power.