Trying to figure out seat tube angle… tried this:
I thought the back hole on the Cervelo P2 was supposed to be a 75 degree seat tube angle, but when I measure from the center of the BB to the center of the back hole, it’s more like 78 degrees, and the front hole 81-82 degrees. The bike is level on the floor (and conveniently resting upon the kids toys), which is verified by measuring the top tube which is at 0 degrees. What am I missing here? Thanks.
Seat tube angle is the angle of the seatube, which used to be the angle of the seatpost. Now with non traditional frames seat tube angle s more commonly (and incorrectly) measured to either the center of the seat or to more or less where the center of the seat would be if it was under you when seated.
You want to measure the angle with the rule bisecting the saddle closer to the leading edge of the the seat tube cut-out (hole). Maybe your iPhone needs calibration 
Those angles are merely estimations. The two holes are on either side of the centerline of the seatpost, a fixed distance off of the axis; so as you extend the seatpost the effective angle from the center of either hole to the center of the BB will change (the back hole will get steeper, the front one slacker).
Unrelated note: It’s amazing what a good bike fit tool an iPhone is. You can measure angles, and take HD video for evaluation.
It looks like it is 75 degrees. Your iPhone may measure in more than one plane, so it will have to be steady in the frontal plane to measure accurately in the sagittal plane.
Ahh, that makes sense. So perhaps that is why, when I got the bike and was fit by someone who probably knows what they were doing, they had the seat in the back hole… and I, of course, thought “that can’t be right”, so I moved it to the front hole. But maybe the back hole, at my seat height, was closer to 78 degrees all along…
The front hole/back hole setup can be thought of as allowing extra sliding along the rails (the bike’s geometry doesn’t change, just the saddle fore/aft). What matters isn’t the angle of the seatpost, it’s the ultimate saddle position (which is the combination of the seat tube angle and the saddle fore/aft). Use whichever hole gets you to the right saddle position.
To measure what angle you ride at, use the phone to take some video on a trainer and measure the angle from the BB to your hip.
What am I missing here? Thanks.
What you’re missing is that in this day and age seat tube angle is an antiquated and basically useless measure. Forget the angles, just use XY coordinates 
Just as we can use “pad stack” and “pad reach” for bar position, one should just use “saddle stack” and “saddle reach” (or “saddle offset”, if you prefer) for saddle position.