Am I Understanding PAR correctly?

[ArthurLK11]

I'm uncertain if I understand PAR correctly, but I think I got it. According to Aquarium Co-op, my plants are medium light plants that require 20-35 PAR. I have the Aquarium Co-op light which at 100% is way too powerful. So I've been running the light at 20% which should give ~35 PAR at the bottom of my 10 gallon tank. The tank is 12 inches tall and with a starting PAR of 177, multiplying that by .2 converts to a PAR of 35.4. Is this right? I want to make sure I'm giving my plants enough light. 

[Beach Cruiser]

Download the Photone app & measure with your phone to get in the ballpark. Be sure to follow the instructions (a piece of paper as a diffuser is required in some applications).

Basically you position the light at the same distance as the substrate will be & it gives you a number. I subtract another 15% when taking a "dry" reading to account for water refraction, etc. 

Mine read out at 30-35 Par which is pretty indicative of what i can easily grow.