is that what the readout can show, or what the probe is claimed to be able to handle?
Btw, high pH is referred to as basic or alkaline - maybe it was just a slip-up on your part mentioning "acidic" in that context?
Tbf, I'd personally consider a pH meter to be superfluous for a simple STB extraction like this, especially given the amount of lye you seem to have added.
I'd focus on the heptane - what temperature was the soup at during the pulls? Gently warming things on a water bath (no naked flames!) to about 40°C should help the rather selective heptane to pull more DMT if it's there. If you hadn't added a vast excess of lye, I'd suggest a higher temperature, up to 70°C even, but in this case it would risk harming* the product.
*i.e. producing a large amount of dark polymer.