OK, I did not see that option for extra concentrate or dextrose but on the description it stated for 3 gallons but enclosed in the box it is 6 gallons. Reducing the amount of added water should be the same as adding extra, correct?
If it's a 6 gallon kit, you should make it to 6 gallons. Shorting a little bit of water is OK, but not 3 gallons. Consider strongly doing it as follows, from the LabelPeelers website:
"*
If you're a fan of the Island Mist Series kits but like your wines a little bit stronger, consider "bumping" them with 1 pound of corn sugar and 1 liter of grape concentrate.
"Boosting" Guide:
The boosting of an Island Mist series kit is easy and brings the alcohol content to approx 9.5-10%. Simply adding corn sugar alone will boost the alcohol of your kit without adding body or mouthfeel to support this increase. Doing this will leave your wine tasting "hot" and unbalanced. The addition of white or red grape concentrate leaves your wine with more balance and produces a more enjoyable end product. Both the 1 lb corn sugar and 1 liter of grape concentrate should be mixed in stage 1 after adding your main juice pack and before topping up to the 6 gallon mark with water."
If your Mist kit has a red wine base (like cab, merlot, pinot noir, malbec, shiraz, etc.), you'd want to order the red grape concentrate. If your Mist kit has a light or white base (like chardonnay, pinot gris, white zinfandel, white merlot, etc.) you'd want to order the white grape concentrate. In either case, also get the 1# corn sugar and go to town. When I've done the Mist kits in the past, when ordering from LabelPeelers, the options to add the corn sugar and concentrate pop up, you just add them to your cart, and you're good to go. Since you already have your kit, you could get the proper concentrate and sugar boost from anywhere you like.