I recall from earlier this year that your engine has a number of modifications including a modern variable advance curve electronic distributor, and that you took your XK120 to a specialist to have it tuned on a dyno.
My XK120 has the 3.4 engine with 8:1 pistons, and the only major modification is an XK140 C-Type head. The distributor is the Lucas 40199 model, which was supplied to both the XK120 and XK140 with 8:1 compression engines. The XK120 Service Manual, page P.5, shows the advance and rpm specs for this distributor.
Note that the maximum advance shown in the Service Manual is 13-15 distributor degrees at 1600 distributor rpm. The distributor turns at half the crankshaft speed, so double those numbers for timing light purposes. Thus 26-30 degrees at 3200 rpm.
Below is a photo of the Shaft and Action Plate found in the Lucas 40199 distributor. Note how maximum mechanical advance is controlled. The two weights mount on the pegs. On the bottom of the weights are small tangs that drop into the two holes in the action plate. The diameter of the two holes in the action plate limits how far the weights can move outwards under centrifugal force. The action plate is stamped “14” which is 14 distributor degrees or 28 crankshaft degrees. I have several of these action plates, each with different diameter advance holes and different degree stampings. The point is that Jaguar supplied distributors providing fairly conservative maximum mechanical advances.
Back to my XK120. I completely rebuilt my 40199 distributor and then tested the results on a Sun Distributor Machine, which showed the distributor was advancing nearly identically to the specs on page P.5.
When I installed the rebuilt distributor in the car, I performed a number of low-speed hill climbs in 3rd and 4th gear to determine the optimum static advance. I settled on 10 degrees BTDC. At this static advance, with 91 octane fuel, the engine gives maximum power with no pinging. So to answer your question, my engine advances a maximum of 38 degrees at 3200 rpm under load, (10 degrees static plus 28 degrees centrifugal, and disregarding the vacuum advance which provides additional advance under low load conditions.)
Last year after my C-Type head was professionally rebuilt, I wanted to make sure the carbs were properly tuned and set-up. So the final tuning was done at a shop that specializes in dyno-tuning British cars. Below is a graph of the results. Maximum HP at the rear wheels is approximately 150.5 at 4580 rpm. Maximum Torque at the rear wheels is 196 at 2269 rpm. And for most of the loaded rpm range, the air/fuel ratio is between 12 and 13.