How to read allergy test results

I work in an asthma/allergy office and we do a lot of allergy testing. I'm confused on how you measure the results. The scale we use is 1+ to 4+ for a reaction. But thats not in mm. I asked my nursing supervisor whose been doing this for over 20 years (the dr even asks her opinion sometimes, she's so good at it.) She just eye balls it. Doesn't use any measuring device. But when I look on youtube it shows the nurses actually measuring it. But I can't quite see what they are reading. Any pointers?
If I were a patient in your practice, I would seriously want to know that the one responsible for reading my (my child's) allergy reactions was very well trained in the science of allergy test analysis.I would truly be appalled if I knew that the responsible party was using tips & tricks obtained from an online forum.Demand an appropriate orientation/education. You deserve it.