I usually just make my mustard concoctions by adding ingredients to taste. Although have something to rinse your palate and try on pieces of bread as this could ruin your palate for the rest of the day if not careful. Sounds good though. Have fun!

Sara and I made mustard - we eyeballed it by putting the mustard powder in a bowl then adding vinegar and wine and stirring until it looked right then adding mustard seed and horse radish- also it thickens up after a couple days so bear that in mind. I also made a plum / orange / lemon / grape mostarda a few weeks ago that came out GREAT. I canned a bunch. Did I write the recipe down? Of COURSE not, but I basically pulled what looked best from several recipes and then used all of the fruit odds and ends in the fridge.

this looks like a good jumping-off point: I'd probably start with less liquid, then add to get what you want as far as consistency is concerned. http://simplegoodandtasty.com/2009/08/09/homemade-mustard-recipe

Are we talking "mostarda" here? It's really more a pickled condiment than a mustard as such, mostarda di cremona being the classic. I've made one using a simple syrup and citrus peels (agrumi) and powdered Coleman's dry mustard.