For our crops (stone fruit) we tend to know the nutrients that are traditionally low, and at what season they are more likely to be low. Applying then is only a guess, but an educated one to make up for the lag time on getting sap results. For us, and I would imagine it would be the same for a leafy green, the main limit is often phytotoxicity, either from a specific nutrient or the net amount of material in the solution. I’d divide the nutrients as macro, micro, and zinc. On a annual crop I’d imagine you do OK with a soil-applied macro. As a foliar I would suggest you seek the best quality chelated form available, and spray at increasing rates until you see a response. A test row, perhaps. Zinc is the one micro we don’t bother to use as a chelate because we can get it in as a mineral spray directly, and is the main nutrient our trees are deficient in. That said, you would likely still need a zinc chelate to keep a low rate/high efficiency spray, since no one wants lettuce that tastes of zinc. We have the luxury of post-harvest applications. I imagine every chelate has a different effectiveness for each crop, based on waxiness for example. Try what you normally use against something else and see which is better. I don’t think it would take more than a couple years with a “head-to-head competition” to see what works best. Testing that control application vs a test row is 2x the money, but will pay for itself if you plan to be growing for the long haul.