To be truly accurate you also need the weight of the solution. Guaranteed analysis are reported as percentages by weight NOT volume. However, since the doses we use are quite small the margin for error is acceptable.
Here's the formula,
Desired Concentration (as ppm) / % of element in fertilizer * 10
So if you want to raise Fe 0.5ppm in 25 liters of water...
0.5 ppm / 0.306% * 10 = 0.1634 ml/Liter
25liters * 0.1634 = 4.085ml
Be careful using this to calculate phosphate and potassium. Laws require manufacturers to report P and K percentages as if they were P2O5 and K2O even though they are not.
For potassium divide the percentage (usually shown as "Soluble Potash (K2O)" )
by 1.2 to give you the percentage of potassium.
For phosphorus divide the percentage (usually shown as "Total Phosphate (P2O5)")
by 2.3 to give you the percentage of P. If you want PO4, which for our purposes we usually do, multiply the P percentage by 3.07.
Remember, the analysis is listed as percentage of elements by weight. So for example, don't confuse total nitrogen and Nitrate (NO3).
The molar mass of NO3 is 62.00501 as opposed to Nitrogen which is 14.00672 which means nitrogen is lighter. To convert Nitrogen to Nitrate multiply Nitrogen by 4.43.
The above formula also works for dry fertilizers. To get the percentages use a molar mass calculator
(or do the math yourself if you wish lol).