Sweet! I have no idea what the best input voltage is. I'd have to look at the THD+N vs voltage and see if there's a sweet spot. If not, higher is better. Same for the source, by the way.
I wouldn't obsess overly over it. Just beware of the limits of your measurement gear. Also, there's value in the simplicity that comes with, say, 10 dB or 20 dB attenuation even if those result in lower voltage than the sweet spot. It's just easier to read things on a graph when you know you have to add/subtract 10 or 20 dB and not 4.887762 dB.
I have an Audio Precision APx555 so I'm using the APx software. I do this for a living (see my signature). 🙂
Last I looked ARTA looked like a good place to start. I also seem to recall that there were some options that required payment, but the payment was pretty reasonable (say $50). Some of those seemed pretty capable as well.
Tom