The evaporater has a volume of about 0.2 to 0.25 litres and as you have a maximum charge of 300grams for the BD50 it seems safe to frost the evaporater to the exit point but if you can weigh the refrigerant in and use no more than 300 gram it would be better. If you put too much refrigreant in then it can ruin the compressor when you start it up from warm.

The BD50 has a cooling power of 160watts at -5C, 124w at -10C, 95w at -15C an 72w at -20C so the cooling power drops quickly as it gets colder and will seem to go slower and slower as the box temp falls. Once the heat flow in matches the cooling performance this will be the maximum cold you an achieve.

If you set the thermostat to say -10C and then when it gets to temp and starts to cycle on and of by the controller - time the period the compressor stays off and time the on period till it trips again. From this we can find out what the heat leakage is into your box and then find out what the lowest temperature you can expect to achieve.

Chef