Most of the pilots will measure pitch by a pitch meter. This is the traditional method used for many years. But there are many factors affecting the accuracy of the measurement.
• - Flybar is not at horizontal level. (Small difference depends on Mixer ratio setup)
• - Blade is not perpendicular to the flybar. (Tiny angle error gives big measure difference)
• - Pitch meter is not clamped on the blade in parallel. (Tiny angle error gives big measure difference)

All these tolerance sum up will give over 0.5 degree error of readings, or even 1 degree for some pilots. This is why we normally use another method to measure the pitch.

Before measurement, we need to have some parameters.
1. Distance from blade hole to tip (L)
2. Distance from spindle shaft to tail boom. (h)
3. DIY a card board / Blade holder for the measurement (As shown in the photo). This will be the pitch meter of yours.
Step:
1. Adjust to the pitch you want to measure.
2. Rotate both blades to the back, on top of the tail boom.
3. Keep the flybar at horizontal level. Measure the vertical distance between both blade tips (d) by your DIY meter.
4. Done. Follow the calculation below, base on the algorithm shown in the photo:

Explanation diagram

You can see, with this calculation, a big measurement error (a few mm) will give tiny difference (less than 0.5 °) in the calculated pitch angle. So, you may feel free to measure the (d) roughly. This will still give you an accurate pitch angle.

One for note is that one blade will go up for +ve pitch, while the other one goes up for -ve pitch. (which blade goes up depends on the delta angle of the head setup)

Small Tips: Construct a table and put it in your purse. Like the one shown. You will get the pitch angle easily, no need to take sin every time!

Small Tips:
Distance-to-Pitch Table
Real Pitch angle over measurement
Besides the accuracy, there is another important advantage for this method. The ACTUAL value of pitch angle. For any practical design, the head will have free play in pitch, because the joint need to be smooth and thus have slop. So, when measuring the pitch (distance d), try to hold the lower blade instead of the flybar or upper blade. This will cancel the free play of the head and gives you the exact value of pitch when the heli is in the air (without loading).
When measuring 0° pitch, try to hold both blades so that the free play gives same (d) for +ve pitch and -ve pitch. And that will be mechanical mid-point for 0° pitch. (Of course, it will be -ve for real value)
Limitations: