Sounds like your boss hasn't either or he wouldn't have hit the lines, lol. Here are a few easy field methods I've picked up over the years. Bet some of these guys have some other good ones too. They are all based on the properties of an equilateral right triangle. Relax, no math involved. Walk out to about where you think the top of the tree will hit. Now, if you were to see yourself and the tree from a side view, and draw a right triangle from you to the trunk, then up to the top of the tree, and then back down to you, if you are standing in the right spot you would have a right triangle with two equal sides. In other words, if the angle from you to the top of the tree is forty five degrees, then you have to be standing as far away from the tree as it is tall, right?
I use three different ways to check this angle. If it is really critical, I have a plastic carpenter's speed square that has a bubble level on one side. I will hold that upand sight along the long side while someone from the side makes sure the bubble is level. If it is level, and the ground is level between me and the tree, and I am looking up that right at the top of the tree, then the top will hit 5-6' behind me, everytime. The 5-6'mis to make up for the height of my eyes, and is a little dependent on the height of the cut, but you get the idea.
A little simpler way that is almost as accurate is to find a four foot or so long stick. Tuck it under your armpit with your arm extended and grip it. Then untuck it and extend your arm as level as you can, and hold the stick up as straight as you can. If you are looking past the tip of the stick and seeing the top of the tree, same thing. It will hit six feet or so behind you. Obviously, this isn't quite as accurate as the square method and takes a little more practice, but will still get ya close.
Finally, if ya just need to get in the ballpark, throw the ole Nazii salute at it, and use your fingertips as a guide. It'll get ya in the ballpark. Keep in mind, this is for level ground, in the hills you have to adjust a little, but the principles still work.
Hope this helps, and who knows, ya might teach your buddy something next weekend. I've won a few cases of beer over the years, it's amazing how many guys have been doing this for years, and still just guess. Jeff