6.2 - 7.0, closer to 7.0 in veg and closer to 6.2 during flower formation....of course you really don't want to do much foliar spraying past week 3 or 4 of flower.
The above is my standard, but if follows the logic outlined below.
From the book: Mineral Nutrition of Higher Plants, by HOrst Marschner, he discusses transloaciton of minerals through leaves.
he mentions: "The small pores are lined with fixed negative charges....Accordingly, permeation of cations along this gradient is enhanced whereas anions are repulsed from this region. Uptake of cations by leaves is thus faster than that of anions and is particularly fast for small, uncharged molecules such as urea. However, when applied at higher concentrations as foliar sprays, differences in uptake rates of nitrogen from urea, ammonium and nitrates become negligible."
From this I would say, based on the latter part of this paragraph, your mineral's are probably high enough to out-compete hydrogen ion (those things that make the pH low). Still, lower pH (which is equal to high hydrogen cations) may compete for or inhibit uptake of positively charge cations (Mg2+, Ca2+, Fe2+ etc). This is enhanced by the fact that H+ are the smallest cations on the market and can access those pore more readily.
There's a flip side to this, those H+ could line these pores and allow negatively charged mineral nutrients to be taken up, like Sulfates, phosphates which help with flower and fruit produciton.
It seems logical to me, that pH may depend on what mineral you want absorbed. If you're going full spectrum treatment, maybe 6.2-7 would be good. While if you're after a particular anion mineral nutrient go lower pH 5.5 - 6.2 and higher 7.0 - 8.5 if want push cations through the pores.
Everything outside of quote is my opinion, though...keep that in mind.
