There is a lot of confusion about fertilizers !

Those NPK figures (maybe topped up with Mg) are just the % concentration of the fertilizer. So actually 10-10-10 or 20-20-20 or 30-30-30 are "the same" fertilizer only their concentration is different.

This results in the fact that only the ratio of the elements between them is important because concentration can be adjusted as needed. For instance a 30-30-30 NPK can easily be used as 10-10-10.

But the real general reference is NOT to stick to those % NPK values but calculate everything as ppm based on Nitrogen. With such calculation every fertilizer can be reduced to a basic type of concentration irrespective of its commercial composition. OK, the ratios stay the same !

Simple reference calculation ? take a 10-52-10 fertilizer (like Scotts). used at 1 gram (or 1 milliliter) per liter water will yield a 100 ppm N solution. Same simple rule for, say, a 4-5-6 NPK ... one gram(or one milliliter) in one liter of water will yield a 40 ppm N solution.

How much to use ? I was suggested by "one of the best" that, for cacti, 100 ppm is a good concentration for regular fertilizing, 150 to maybe up to 200 a season's boost in Spring. .. but everything can be easily adjusted with this calculation trick ...

For Lithops I tried 50 ppm but that is far too much, even 25 seems plenty ! It may be clear that with a fertilizer, high in N, one cannot make a 25 ppm dilution even in a 10 liter bucket it is kind of tricky ...

BTW The according P and K concentrations cannot be directly calculated in this way because their analytical concentration is given as P2O5 and K2O ...

I made an excell speadsheet for this but the only important parameter is Nitrogen anyway ...

An important side factor to consider however, is salt build up especially for P and K as N is washed away ... also keep an eye on the Ca/Mg content as those tend to be converted into their corresponding carbonates ...

.. and of course one can just go on with the regular "splash" of fertiliser if calculating is a pain ...