Pickled/preserved lemons/limes - grams of salt per kilogram of fruit? [
I think this question here was originally too unclear.
because I was still researching and figuring out exactly what I wanted to ask when I posted it.
I edited it a few times,
but I think my editing has changed the question enough that it's no longer clearly the same question as when I first posted it,
so I decided I should probably post a new, separate one.
]

I make sauerkraut with 20 grams of salt per kilogram of cabbage.

Is the proportion different for pickling lemons/limes?
I would guess maybe lower because of the citric acid already there...?
I found one site saying 5~10% for "North-African style",
which is what I think I'm going for.
(I used to get them at Morrocan stores,
but the city I live in now doesn't have any.)

But if that "5~10%" is really by weight in the same way,
then that's a lot more than the 2% for sauerkraut,
which is surprising and confusing.

Also, I don't add any water to my sauerkraut,
but maybe that's necessary for lemons/limes?
In which case I would just salt the water at the same salt/water ratio as the salt/fruit, I assume?

The ones I used to get, I'm not sure, but I think they used nigella.
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.