Originally Posted by rpenmanparker

Not necessary. Here you go:

The way these measurement are done is not in relation to each other, but in relation to a fixed point in space. It is much easier that way as you don't need to use a level, plumb bob, or other tool to make sure you are not making a mistake. Here is how it is done:

You first inflate your tires to normal pressure. Then using supports or a trainer stand or an assistant place the bike with its rear wheel against a wall and perpendicular to it. The bike must be perfectly upright and not slanted to either side. Also must be level front to back. For stack and reach also insert a seat post in the seat tube and mark the point on it that is at the exact same height from the floor as the center of the top tube where it intersects the head tube.

Then you make four measurements:
A) Wall to marked point on seat post.
B) Wall to top center of head tube (not to the top of headset).
C) Floor to center of bottom bracket vertically.
D) Floor to top center of head tube (not to the headset).

You really don't need to make sure the measuring tape or stick is PERFECTLY horizontal or vertical as the small errors due to slightly slanting the tape or stick are negligible. Eyeball should be fine.

Now you have everything you need to relate the three defining points to each other. D-C is the stack. B-A is the reach.

Done and done.