Doglips, the amount of salts is negligible, or it should be. When I foliar spray, I don't go more than 1/2 tsp/gallon and sometimes use rainwater as a carrier as my well water is super hard.....high in salts. If you go more than 1 tsp/gallon, you risk foliar burn. I extrapolate those amounts whether loading a 40 gal. PTO driven air blaster or a 4 gal. backpack sprayer.
I also don't foliar spray if I expect hot temps with low RH. I'm gunshy when it comes to foliage burn.
Question, what is the problem with fertilizing mangos during fruiting? I have 2 small trees, Pickering and Mallika, loaded with fruit and expect to harvest soon. I recently drenched them with a Peter's 20-3-19 and they're doing fine. If anything they probably need more K now, at least on a technical, botanical basis.
Mark