Thanks for the update Bill. The behavior that you see is not unusual. It is probably due to Iodide concentration on the different grains, or on the absolute concentration. If it is the latter, then the sensitivity will gradually shift towards being all red if you up the concentration slightly. But, OTOH, either mechanism is ok. It really does not matter. Repeatable success is what counts.
Best wishes on the surgery and may your recovery be swift and complete.