Actually, the teaspoon measures are not nearly as accurate. That said fertilizers are like hand grenades. All you need to do is get in the ball park.
There is quite a bit of math to do what you want. It's easier to use a nutrient calculator. If you want to use one you can use this online version
or download my Windows version
I'm adding how to do them manually just in case you or others would like to know. There are various formulas depending on the measures but this is the basic math to accomplish this.
Parts per million is one milligram per liter. So if you add say one mg of Potassium to one liter you have 1ppm of Potassium.
Here's the problem. Our fertilizers are not elements like potassium. Instead, they're a mixture of elements or molecules.
Take K2SO4 for example. It has 3 elements. So wee need to know the percentages of each. I won't explain how to calculate that. Instead you can use a molar mass calculator
to see the numbers.
If you enter K2SO4 in that calculator you will see we have the following,
Symbol Element Atomic weight Atoms Mass percent
K Potassium 39.0983 2 44.8737 %
S Sulfur 32.065 1 18.4008 %
O Oxygen 15.9994 4 36.7255 %
Potassium is 44.87% of K2SO4. That means that 1mg of K2SO4 has 0.4487mg of potassium (move the decimal to the left two places).
That's all we need to make a calculation.
Example, We want 15ppm of potassium in 10 gallons.
First convert gallons to liters,
10 * 3.78541 = 37.8541 liters
Remember K2SO4 has 0.4487mg of potassium per miligram so,
15 / 0.4487 = 33.43
So we need to add 33.43mg to one liter of water to get 15ppm
10 gallons has 37.8541 liters so,
37.8541 * 33.43 = 1,265.5 mg
So we need to add 1,265mg to 10 gallons to increas K to 15ppm.
It's also necessary to convert forms of nutrients. In this hobby we use NO3 as nitrogen and PO4 as phosphorus. So we need to remember to convert those numbers. There are tons of conversion charts available.
Example, if you want 7ppm of NO3 you need to convert to nitrogen. The conversion factor is 0.22.
So 7(NO3) * 0.22 = 1.54(N)