Pickled/preserved lemons/limes - grams of salt per kilogram of fruit?
I have experience with Indian lemon pickle, not north African. 

The recipes I use don't give exact ratios for the salt. Salt is added to taste. A couple tablespoons per quart of packed lemon is normal.
Lemon pickle is very salty which besides regulating the fermentation also helps with the bitterness from the peels.

You don't need to really worry about the exact ratio. It is so acidic and salty there is little that can grow. I've not yet had a batch spoil- even after months in the fridge.

Don't add water. The lemons need to be covered, of course, but use the juice from a couple extra fruit instead. No sense in diluting the flavor and acid.