So your personal output is off a larger basket (tripple). Have you tried to stuff in 18g into your spouted double? If it doesn't fit with reasonable tamp, then you'd need to scale your recipe to the lower dose.
Huh? I'm not sure why the OP uses a tripple basket for 18-grams; but there's no 'stuffing' involved when using a doppio basket. 18 grams just fits into my stock double basket. Admittedly it's full, but there's still a bit of head space. In my 18 gram VST basket 18 grams fits with a plenty of of extra room. A tripple basket (21+ grams) can't be used to pull splits, as most tripple baskets won't fit into most standard portafilters, including mine. So, the OP will be forced to use a smaller basket. In any case, as I said, I do it all the time and use the same dose, 18 grams, as he does. If I were he, and his stock doppio basket is too small, I'd get an 18 gram basket that fits his double-spouted portafilter and keep brewing the shots as he usually does.