You have to distinguish between the measurement with the iPhone-App and the optical measurement on the computer. Those are two different types of measuring and therefore you have to set the points from where you measure differently (thats why I measure the actual value from baseline to peak).

In the second measurement, there were two peaks close together. In this case, I just took a point between those two peaks. Of course, this is a measurement error, but this error is quite small, just test it out yourself !

In the third measurement the peaks aren't that distinct, but you can still see them. Just search for the peaks that have the greatest amplitude. That's not arbitrary.