Good to hear your soil has sufficient reserve. If you just need maintaining, 1 table spoon for 100 sq ft every other year is enough. Dissolve in water and apply evenly with a watering can. When plants transpire and uptake the boron, the excess will accumulate at the tips of the leaves and when it rains, it washes out and recycles back in the soil (drip line!). In our climate, toxicity is unlikely because of the high rate of leaching.
For me, I did 5 applications each of 4 lb borax on one acre from Feb to June last year, based on my local rainfall/temperature pattern. So about 2 lb of net boron per acre and 3 months prior to that I already put down hundreds of pounds of lime and gypsum to raise calcium level. It was kind of high rate because I need to get it deep in the soil profile. I don't need to this year. My oldest peach tree finally has hundreds of flower buds surviving -12F and multiple warm-ups and freezes, after 3 years of nearly zero harvest. It might be able to fend of plum curculio without any protection as well.