I got it working....
Using the 'uint32_t analogReadMilliVolts(uint8_t pin)' ability, combined with a map routine.
All FINALLY seems to work as planned.
Just got the slight issue of thermal shutdown now 
Not going to fix that.
The TLS850B0TBV50 3.3v regulator is good up to 40v, but I only ever planned to throw 50mA at 10v through it. That is all fine (10v is the supply on the 10 way ribbon cable).
But.... the 20 way ribbon cable was assumed to be the same 10v supply. Nope.... 24v DC 
(I am making this for someone and the information is sadly lacking).
So at 19v.... it thermally shuts down and I am not surprised. No space for a heatsink, and I don't think that would help anyway. I am adding a small 25mm, 5v fan to help in general.
But... dropping 24v to 3.3 just isn't going to happen and I can't think of a way around that.
There is a hefty diode on the reverse of the PCB for polarity protection.
Maybe a Zener arrangement here could help, but I think that just throws the heat from the regulator to the Zener.
.... Hmm. Always something.