thats really confusioning. the way i see that, is that there is 1 gram in 5000mLs, or 1gram in 5 litres. so that would be 20mg in 1 litre. convert that to micrograms, and thats 20,000 mcg in 1000mls. so thats 20mcg in 1mL.
so if u wanted to give 300mcg, you would give 15mLs.
im not sure if thats correct, but thats what ive come up with.