Using an analytical balance (0.0000 precision) to measure µg (micrograms)?
As others have said, that mass is too small to measure with a standard analytical scale. There are two options you could use:


Make a larger volume of your 200 ug/mL solution. 
Make a concentrated stock such as 20 mg/mL. This a 100X stock; you can make 1 mL of 200 ug/mL proteinase K with 10 uL of stock and 990 uL of buffer. Small volumes are easier to measure than small masses (with a micropipette).