Measurement of negative DC voltage
This is an interesting approach. Not one I've seen used, but I suppose it doesn't come up super-often.

You should divide the high voltage down, then use the Thevenin equivalent resistances to make sure your summing resistances are equal (or understand the conversion factor).







simulate this circuit &ndash; Schematic created using CircuitLab

This takes your -30V signal and divides it by 6. Then you calculate R3 to have an equal impedance to the resistor divider (Just R1 parallel R2) which makes this a 1:1 summing node.

I think this will work. You might want to prototype it though! Oh, and use a rail-to-rail op amp. Otherwise you'll lose some values at the top and bottom.