Have you read this article: http://ceramicartsda...file-glaze-fit/
It says it more succinctly than I could. I like the tight pants versus baggy pants analogy.
For your flux, I would try one of the low expansion ones in place of the whiting. Magnesium from talc, dolomite, magnesium carb or lithium from spodumene, a lithium frit if you can get one in GB that's not to expensive, or even some lithium carb with some source of magnesium. Keep an eye on the alumina to silica ratio. I don't fire ^10 so hopefully someone else can chime in here and give you a hand with other suggestions.