A bit late for this .... But to read an off the chart level using your existing kit, you can do serial dilution.
You take the tank water, dilute it 50% with pure water. if you can take a reading, just double it to get your actual result. If your reading is still off the chart, you dilute it again. <Now it is 25% of original so you multiply the result by 4.> Eventually, you will get to a point where you get a reading. You just have to keep tract of your dilutions to apply the multiplier.
80 gal FW with 30 gal DIY wet/dry/sump.
9 fancy golds, 1 hillstream loaches, 1 rubber-lip pleco (C. thomasi), 3 SAEs, small school of white cloud minnows, planted.