I have always measured from the green back to the tee.
I hire a surveyor to find the exact center of the green, and then shoot distance to find wherever 150 yards lies in the center of the fairway, from the green center. This involves laying a tape from each side of the fairway edge and finding the middle of the fairway. You can't do this too easily without a prism. From there we set up a stake and measure back to the center of the various tee quadrants. For instance, if three blocks are on a single tee, you divide that tee in to three equal sections, and measure from the center of each of those sections to the 150 stake. That is how I have found the angle from each tee block.
I have rarely encountered a situation where the 150 stake is not in an obvious turning point, except on double dogleg par fives that are tree lined. In the two instances that I have encountered that, I had the golf pro pick another turning point, and we set up the second turning point for measuring back to the tee. That's where it gets tricky - on the par fives that are tree lined.
On the par threes you can measure from tee to green.
Funny story: on the double dogleg par five that we had just measured, John Daly filmed a Titelist commercial with Michael Jordon. He dropped balls on the fairway a long ways behind our second turning point and proceed to drive them over the trees on to the green. The golf pro and I looked at each other and just laughed. This was back in 1995 when he was in his prime. Those balls had so much elevation to them, it was unbelievable.
Turning points are kind of irrelevant with those kinds of hitters.