This question cannot be answered sensibly. A millilitre is a measure of volume, with dimensions [L3]. A milligram is a measure of mass, with dimensions [M]. The two measure different things and basic dimensional analysis teaches that you cannot convert between measures with different dimensions such as these without additional information.
As a simple mental exercise consider a millilitre of air and a millilitre of water. They will have very different masses.
620 (mg per liter) = 62 mg per deciliter.
There are 109 mg per dL.
1000mcg per mg
0.1cg per milligram.
One dram is 3,888 mg
10200mcg (1000mcg per mg).
about 148 mg per grapefruit.
1000 mg per gram
250000mcg (1000mcg per mg).
1.5mg (1000mcg per mg).
400mcg (1000mcg per mg).
No, 300 mg per cc does not equal 25 mg per kg. To convert 300 mg per cc to mg per kg, you need to know the weight of the individual in kg. If you're looking for a general conversion, you would typically need to know the volume of the substance in cc and the weight of the individual in kg to determine how many mg per kg 300 mg per cc would be.