How to convert ppm to percent:
1% = 1/100
1ppm = 1/1000000
So,
1ppm = 0.0001%
or put another way 1% = 10,000 ppm
The directions say that calcium is 2.57% so that means in one of their recommended doses at say:
300g/10 kg soil you are adding 25,700 ppm of calcium. Which is the same thing as
2,570 ppm in 30/10kg or
257 ppm in 3/10kg or
25.7 ppm in 0.3g/10kg soil, or
2.57 ppm in 0.03g/10kg.
Since there are 1000 mg in 1 gram that means 0.001 grams = 1 mg. So you end up with 0.257 ppm in 0.003g/10kg or 3 mg of that fertilizer will give you 0.257 ppm when you add it to 10kg of soil. Since 1 liter of water weighs 1 kg that means 3 mg in 10 liters gives you 0.257 ppm calcium. Or put another way 1 mg of that fertilizer will give you 0.0865 ppm in 10 liters of water.
If you like using gallons instead then that is 1 mg in 2.64 gallons gives 0.0865 ppm calcium.
So finally to answer your question that means that 10 mg in 2.64 gallons (or in 10 liters) gives you 0.865 ppm calcium.
The problem with using that fertilizer is that you do not know how much of it to add for aquatic plants. Look at their packet, you will see that they recommend different amounts for different types of plants - 300g/10kg for fruit trees but only 150g/10kg for palms. Since we don't have that advice for aquatic plants I really don't think it is wise to use this fertilizer for your plants.
However if you really want to experiment (and keep in mind it will likely cause you issues), then you can sort of get an idea of how much to add by taking a value like calcium and figuring out how much of their stuff gives you an appropriate level of calcium for aquatic plants.
Since calcium is generally recommended in the 10-30 ppm range that means you'll have to add 115.6 mg (or 0.1156 grams) in 2.64 gallons (or 10 Liters) to reach the 10 ppm calcium minimum recommended range.
Again, I really don't think that fertilizer is suitable for aquatic plants. Fruit trees and food crops need much higher levels of fertilizers than aquatic plants do and also need them in different ratios than aquatics do. I think using that fertilizer will get you into trouble.