This is really cool, and very impressive. I think it's well within my comfort zone for DIY stuff, and would be fun to build. However, I'm really glad that I got one from you because you were able to calibrate it with a real PAR meter. That's the questionable part to me. Sure, we could pick a known light source that should read a certain PAR at a certain distance, but you're assuming very little or no variability in that light source, if you are shooting for relatively precise measurements.
That said, perhaps that amount of variability doesn't matter for the purposes of how we use it on our own tanks. I mean, if it was off by 10%, that would only be off by 10 PAR at 100 PAR reading. And for what we do, that's not a big difference in real world terms. It would seem that the more important thing would be if, say, you were measuring plant growth in your own tank or trying to change lighting due to algae, that adjusting the lights up or down to change our PAR reading by 25% or whatever, that you have a known starting point, and a way to measure from that point, and can get back to it, or adjust it further.
In other words, it doesn't matter so much that I am getting precisely 70 PAR at the substrate so much as if I want to lower my lighting, I can lower it by a certain measurable percentage, observe changes, and make further adjustments. So maybe it's not too important to get hung up on being incredibly precise, but rather simply trying to get as close as possible, and that's good enough and super useful for hobby use.