Start this test while the system is mK. Close V1 and open V4. This will divert the He from being pumped back into the MC and into the dumps. The fridge will continue to stay cold for the duration of the test. When the He runs out, the cooling power drops, and the MC temperature will spike and hold around mK, the pressure in the dumps will rise much more slowly until the He runs out as well.
The value of P1 when the slope decreases suddenly and the temperature rises rapidly, gives the amount of He in the system.
To end this test, you should collect the rest of the mixture.