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For dual gate mosfet mixer, probably RF-to-LO port isolation is about 20db, LO-to-IF port isolation 30db. Because of the high impedance, the DGM has a very narrow bandwidth and low conversion gain for image noise. Now I think this measurement is a moot exercise purely for learning purpose. I will probably change it to a "thought experiment" without doing all the troublesome practical work. It is a lot of hassle to measure TOI with all the jazz of diplexer, Pi network, If-transformer, crystal filter and frequency source BPFs.
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In my previous tests with a 134MHz LO and a high side LO at 156MHz, I used a broadband 4:1 step up transformer to get double the voltage to the G2 pin of the mosfet. My sig gen had to run at about 13dBm output to get good conversion efficiency even with the transformer in place. If the sig gen noise floor at 10.7MHz offset is (say) -145dBc/Hz then the absolute value is 13 - 145 =-132dBm/Hz.
If the mosfet has 20dB L-R isolation then -152dBm/Hz can be assumed to be leaking to the RF port. This is over 20dB higher than thermal noise. This is why a regular commercial sig gen will often spoil any attempt to measure the noise figure of the mixer if the sig gen is used for the LO. I have demonstrated this in the past and some of my sig gens degraded the noise figure of the mixer by over 20dB. Adding a narrow BPF at the sig gen output cured the problem as the filter had well over 20dB rejection at a 10.7MHz offset from the LO frequency. There can also be wideband noise floor issues from the sig gen way down at 10.7MHz and also at the image, but the main problem seems to be noise on the LO at the RF frequency.