Converting to Units/mL with a varied ED50?

I'm trying to determine how much IFN-y to add (of a 10mg stock) to my BMM infections.

I need 200 Units/mL and the stock IFN-y has "An activity range of 0.3 - 3.3 x 10^8 units/mg, encompassing an
ED50= 30 - 300 pg/mL".

What would be the best way to calculate this?

Please help!!!
Update: Stock concentration is 200ug/ml