I have a rangefinder but when I am hunting open fields I grab at least three to four sticks and put them out at 40 yards in a semi circle. I know if that turkey is within that circle I know he is dead. The stick idea has worked well for me in the past and I will use it in the future. You could also make a hole in the dirt or scrape away some grass heck any sign that will let you easily tell a certain specified distance. You could also bring with some yellow tent stakes with and put them in the ground.
"Strength does not come from physical capacity. It comes from an indomitable will." --Mahatma Gandhi
"Never confuse a single defeat with a final defeat."--F. Scott Fitzgerald, American writer
i just rely on experience to judge distance and i always tell myself when drawing a bead ..."if you think he's 50 yards away, let him get closer." i shoot a 3-inch, 12 gauge and have killed them at 50-55 yards but try not to take those shots anymore. if they hang up at 50, usually a soft purr will bring him a few steps closer. but if he's comin, i let him keep comin. i'm addicted to that rush of watching the tom put on his show as he gets closer and closer...