Ugh Math ! LOL
Starting with your first post: in US measurements
If the directions say for a mix at 0.012% = "8 fl oz per 50 gal" then we need to find how many ounces per gallon.
50 gallons divided by 8 ounces = 0.16 oz solution per gallon
1 Teaspoon = 0.166666667 (1/6) Ounce
1 Teaspoon = 4.92892159 Milliliters
The first is a normal dose. The second is 2x dose for severe. The third is 1/2 of normal dose. So the answer is as follows:
0.012%: .16oz solution per gallon = 0.96 tsp = 1 tsp or 4.73 ml - for normal solution.
0.024%: .32oz solution per gallon = 1.92 tsp = 2 tsp or 9.46ml - for stronger solution.
0.006%: .08oz solution per gallon = 0.48 tsp = 1/2 tsp or 2.36 ml - for the remainder of treatment.
Now to work on the second part of your query.