Hi Chris, G4SDG,
It will be interesting to see your measurements on the LimeSDR Mini. The two-tone signal I'm using, is the default "Tone" button on SDR Console.
The 90% drive level is based on default SDR Console setting - and it also gives me 6 dB back-off.
Some time ago I measured LimeSDR Mini output vs. drive setting in % and came up with the following curve measured at 2.4 GHz and for levels between 50 to 100%. Blue line is output and orange shows stepsize. (see also DD1US homepage under "What's new" of 20'th Nov. 2019)
Output increases smoothly most of the time, but there are jumps of 2 to 4 dB at certain %-settings - which can make output adjustment a bit jumpy! Especially from 87 to 88% where output changes by 4dB. I don't know if all LimeSDRs behave the same? - maybe you could look into this?