How to measure the height of a tree limb.
1) Toss a piece of twine with a weight attached to the end of it over the tree limb and let it drop to the ground. Place the other end of the twine on the ground and mark it. Pull the twine down from the tree limb. Measure the total distance of the twine up to the place you marked. Then divide the distance by two and you will have the exact height of the tree limb.
2) Take a digital picture of the tree branch with a broom handle leaning up against the tree trunk. (Be sure to include the tree limb where the swing will be hung in the photo) Snap the picture and print it out. Determine the actual height of the broom handle and then figure out how many broom handles it will take to reach the tree limb. You should be able to come pretty close to the exact height.