To answer your question, -35-40 is what I have measured using an SDR Play RSP2 and the spectrum analyzer software (this is the only tool that I currently have that can get a low enough resolution band width RBW to make the measurements). I did use a two tone generator on the mic input to verify the audio vs carrier. Simple matter of turning on and off the tones while keeping the uBitx in transmit mode.
There are two possible causes for carrier leakage that I have fund:
1 - mixer diode issues as you have pointed out
2 - BFO out of alignment
I found that by changing the BFO setting I will get some carrier injected back into the signal, even if the mixer is OK.
I tested the mixer diodes by injecting an ac signal into the BFO transformer across the output coil coming from the BFO to the mixer and measuring the common connection of the mixer diodes with an oscilloscope to verify a half wave looking signal. If they were bad, should not get a full wave rectified waver form. Not sure if it is the best way to verify the mixer diodes, however seemed to work for me.
NOTE: in doing the transmit test I first established the power level at frequency using CW and an SWR/POWER meter in the line between the rig and the dummy load. I then adjusted the tone generator output to be at the same or slightly lower level, and that the distortion on the measured spectrum was minimal. As Ashhar stated, there are audio programs or recorded wave files that can provide the constant 2 tone audio. There are also good kits that can provide a very pure tone/tones.
Also of Note: I am not at my QTH so cannot get to the actual rig and verify the connection points for the mixer diode verification test.