Get yourself a biltmore tree scale stick. You can find them at Baileys-online.com
As for scaling a 100' tree don't buy it that way there is a point that even the forester will stop. I bought state timber where some of the white pine that was 30" DBH was only scaled for the first 50'. I did get 80-90' of log out of some. But if you pay for every last foot you will loose. Take in to account wind shake, insects,bad knots bad butts and loss from felling. Not to mention cost of harvesting . Most mature oaks around here depending on dia. are measured by 16' logs most have one some have a log and a half some two logs and a rare ocasion3 logs but that is rare. the biltmore stick I have measures tree volume buy 16' logs