I have been playing with making FM-synths. What exactly is the scale for FM control voltages? For example, I have been running the square wave output from one synth into an attenuator, before running it into the FM CV port. I gather that the amplitude of the control voltage is controlling the range of frequencies in the second oscillator, but what exactly is that range? If I ran 100% of the signal from the controlling oscillator into the second oscillator, would that be an +/- an octave. Ideally I would like to tune these ranges(which is why I was using an attenuator), I am having a hard time getting a feeling for what those are. I guess I could set up an LFO square wave to figure it out.
That depends … 
I strongly recommend watching Omris Videos on FM-Synthesis:
That should kick-start your FM-Synthesis.
I built an FM-Synth in the style of the Digitone Sound Engine, you can find the patch here:
As @mosphaere is indicating, depending on what you want to do, linear Fm is much better for tonal stuff than exponential. Should all be in Omri’s video.
So what are the units for the fm knob? It says percentage, I get it is exponential, but what is the base? Also, 1% doesn’t seem like a factor of two. It seems like 40% is pretty close.
It would depends on the exact module you are using, which you don’t specify. In general there is a standard for the main Cv, but afaik there is no standard for modulation.
I see what you are saying. I picked it up I think. In the VCV OSC, the fm corresponds to a percentage of the input voltage. However, other modules such as sequencizer 16 let you specify voltages, and from watching various VCV videos, I picked up that a general standard is one volt per octave. From there I can use musical intervals to get variations on the other notes. EG 1.5 volts will be a fifth above the octave.
I think that is about right its tough to figure out for sure though.
Yes, there is a standard for the main CV input. it is 1v/oct. here’s the section of the manual that talks about that: VCV Manual - Voltage Standards
The, as you say, most “FM” inputs will scale the input then apply the same 1V/oct. But the scaling will be much different between VCOs.
As to a fifth, that’s actually an interesting question. for an even tempered scale, 1V/oct translates to 1/12 volt per semitone. So the G an octave and a fifth above C will get 1 + 7/12 volts. But the “true” pitch of the third harmonic of C will be at somethere else.