You are missing some basic understanding of volumes and weight.
Mg is a unit of weight.
Ml is a unit of volume.
If there was a mix of vinegar and water then they would both be volume and the amount (of either) per ml would be dependent upon the ratio of the mix.
There is 50mg of weight, suspended in 1ml of volume. Therefore if you have a desired amount of weight of active that you desire to consume you need to determine the amount of volume you need to achieve the amount of weight you want to consume.
50mg/1ml
1.2ml (of volume) x 50mg/1ml = 60mg/1.2ml
60mg/(50mg/1ml)=1.2ml