Hi buddy, what amount of water were you thinking about and what fertilizer are you using?
Calculating is really simple.
If the dosage recommendation says on the back:
50ml of fertilizer for 10L of water
Then that means double, so 100ml of fertilizer for 20L of water.
If you need more, 500ml of fertilizer for 100L of water 😉
The same goes the other way; if you calculate for 1L, then from 50ml for 10L of water, it comes to 5ml for 1L of water.