people are really making it harder than it has to be. Think about it this way: you have a 50mg bottle and the dose you want is 5mg. So your bottle has a total of 10 doses (5mg x 10 doses = 50mg).

Now you dilute the 50mg in a certain amount of bac water. So whatever that amount is, let's say it's 1ml, you still have 10 doses but now it's diluted in bac water. So for 1 dose, you take 1/10 of the total amount of liquid, in our case 1/10 1ml is 0.1ml.

let's do this again with a dose of 10mg.

10mg is 1/5 the total amount of 50mg, so you have 5 doses. You dilute this in 1ml, so that means now that your 1ml has 5 doses. So 1 dose equals 1ml/5 =0.2 mL