I agree, rangefinder is the way to go. I take mine with me all the time, even scouting or just walking the woods. Maybe if I use it enough I won't need it? I found a new Bushnell Yardage Pro 1000 w/arc on ebay for $275. Works great hunting from a stand also. I can mark everything as soon as I sit down.

I use a rangefinder from my treestand (archery hunting) or on the ground to range obvious shooting lanes, trees, stumps, edges, landmarks etc.. That way when game appears, i know the aproximate range. I have never used it on an animal i was planning to shoot for fear of being caught. I will use it when bored and guess ranges to see if my judgemant is any good.
Rifle hunting i rarely have oppurtunities at shots outside that Maximum Point Blank range (just inside 300yds for my rifle) so i don't usually carry the rangefinder. If i'm hunting open terrain i take it.

I use a Bushnell Scout with ARC. you can select between archery and rifle. You can also select a ballistic value from some supplied figures and it will provide with a pretty accurate hold over for the range of your target.

Rangefinder would be the best solution, but if you're tight for money, try all different scenarios.

I really want to get a rangfinder and would suggest for you to get one. Do try to shoot at different angles, ranges, elevations,and on your knees too.