What we consider as the normal rate is all based on experience and experiment. I still feel that phytotoxicity is the main limit on the high end, and to stay at maybe half of what the plant can handle as a maximum. We tend to have our own limits and then bump them up or down based on need. If I know young trees tend to be very short of zinc, for example, I will do maybe 50% more than our standard, and perhaps 50% less when it looks like a slight deficiency based on a test.

I think you aren’t thinking about the ppm correctly though. If you are trying to match the level in the plant you would have to assume that your spray volume is equal to the volume of sap and/or plant material, which won’t ever be the case. Plants will disperse their nutrients, if only locally, even if they are not “mobile”. Boron for instance can create compounds that make sugars more soluble, and help them move to the growing tips. That is why excess boron always shows up at the tips. It is also part of the cal bond, and those are structural so it makes sense that they don’t move. Try to think of what it will take to get your levels right rather than match the ppm of the plant itself.