Thinking about how most soils work (Promix and Coco included), after a month or so, I find that the "channels" develop thru the medium, leaving a lot of it untouched, or minimally touched. I know of many growers, myself included, that have found what looks like untouched or uneveloped areas deep in the root areas. Any water or nutrients will follow thru these channels, which are simply the areas of the least resistance to water's movement, and deposit the majority of their contents along these channels. Measuring runoff would really only check the areas thru the channels, which might explain some of the measurements that don't seem correct.Runoff is only useful if you can get the water to travel through the growing medium and not around it and through channels. If it travels around the growing medium and not through it you’ll pick up mostly salt buildup on the container which will throw off the readings. Plus there’s the issue of which part of the runoff do you test? The start, after running a gallon or so through it or later? By testing the soil directly you’ll get an accurate measurement that you can then make adjustments with.
One useful tool is to buy nectar for the gods Olympus Up and Herculean Harvest and use that as a flush every couple weeks. The calcium binds to the excess nutrients and renders it null. It also flushed out excess salt buildup and delivers a boost of phosphorus to help with nutrient uptake. It’s cheap and is useful to have in your arsenal.
Hydro growers wouldn't have this problem, but virtually any medium will compact itself slightly and develop these channels. I don't really know how it can be used to remedy or fix this Ph problem, but the comment about the channels developing and possibly skewing test results for Ph or PPM/EC. Plus, the morning medication to control nausea got my mind thinking!
.....Now I'm just thinking about whats going to happen when all of that lime gets working....