isn't this suppose to be 20ml Per dose instead of 25 (bold letters)?? unless this is something else, i have no idea and please correct me if this is a mistake or something.
The 25 is how many doses are in 500ml, 500/20= 25
We first need to calculate a single dose then multiply times 25 for a solution.
Here is the math for the molar mass method. Rather than lookup all the masses for each element then make all the calculations just use a molar mass calculator
Mass composition for Magnesium Gluconate (MgC12H22O14)
Mg 5.8623 %
C 34.7634 %
H 5.3485 %
O 54.0259 %
50 gallons is 189.271 Liters
Reduce the volume to the percentage we want to add which is 0.2ppm
189.271 / 0.2 = 37.8542
So to raise anything to 0.2ppm we need 37.8542mg
Since magnesium is 5.8623% of the mass we need to calculate how much we need to raise Mg to 0.2ppm
(X) * (.058623) = 37.8524
X = 37.8524/0.058623
X = 645.72267
So we need 645.72267mg of this to raise 50 gallons of water 0.2ppm
Since there are 25 20ml doses in 500ml we multiply times 25 to give use the total amount to add
645.72267 * 25 = 16,143.0667mg
So adding 16,143.0667mg to 500ml of water will create the solution.
If you do the math NOT using molar mass the result is similar. However as Jeff said there is rounding done so the total is not as accurate.
37.8542 / 70 = 0.5408
0.5408 * 1300 = 703.0066
703.0066 * 25 = 17575.1643mg
As Darkblade said, these chemicals are probably not 100% pure. So technically neither calculation is dead accurate. However, both are close enough. So to answer your question Happi, Yes those numbers are as accurate as can be expected.