Hi Michael,
I welcome you working on getting a measurement scale. One suggestion you may want to consider would be to mark one of the main horizontal line darker. This would be the datum. Then when taking the picture you might want to align as best as possible with a known gun feature, possibly the center line of the bore. This would help get a true perspective of the measurements to be taken. You may then want a vertical datum line that say locates at the trigger or another known gun feature. Just my thoughts