Unfortunately, the DB numbers in the display are relative, and not actual for low power input like this. Instead, I tried to make measurements using my oscilloscope. My scope doesn't have a 50ohm input, so I wired it up through a splitter and attached the ends to my scope probes.
Feeding the output directly into my oscilloscope, I could see that it was on frequency, and generating 2.46v peak-to-peak.
Output power is measured as: P = Vrms^2 / ohms. We have 2.46 peak to peak. Vrms = Vp2p/2 * .707. So, Vrms = 0.870v. The Si5351-b outputs at 50 ohm impedance, so P = 0.870v^2 / 50ohm = 15mw. That's curious, as other Pico tracker builders are reporting an output power in the 20mw range. It's possible that all the harmonics are accounting for the difference in power. I'm not sure.
The Adafruit protoboard has it's own LDO regulator, and will operate anywhere between 3.3-5v. During my testing, I was feeding it 3v3. With my multimeter, I could see it was providing 3.11 volts to the 5351 chip on the board.
My tracker will be running at 2.5v. Presuming (and that's a big presumption) that the output power will be proportionally diminished, then my board would be making (2.5v / 3.11v) * 15mw = 12mw. That seems pretty low. I'll want to do more experiments with my own board when I get it together.