To reach base temperature turn off the heat load on the Mixing Chamber, and keep the heat load on the still around 4mW. The heat load on the still is required to keep the cooling power high. The optimal heat load on the still to reach base temperature will depend on the cold mass. For an unloaded MC, it was found to be mW, but you may need to tweak this value to find the true base.
thermometers will lose accuracy around 50 mK and will
bottom out completely at
mK. Below this, you will need an
alternate measure of the temperature.