Sadly I've not tried this substitution myself so I'm not too sure about the ratio but you can always add a little and then taste as you go along. (deliciouslyella.com)
If you can't find them, you can make this same recipe with traditional lemons, just taste as you go in case you need to increase the amount of sugar. (theredheadbaker.com)
I really didn't measure the marinade ingredients, so taste as you go until you get to a flavor balance you like. (weheartfood.com)