What is the ideal ratio to mix mustard powder with flour

Dijon has a strong taste from the liquid they use in it (it's not quite vinegar, not quite wine). I'd probably try a blend of some other mustard + a little white wine vinegar (not white vinegar; white wine vinegar) or champaigne vinegar or apple cider vinegar. (Maybe even a dry white wine, if you have that on hand).

If you have dry mustard powder, and some time to let it sit, you could also try making your own; most of the recipes online seem to use a dry white wine as their base, a few use white wine vinegar.

Yes, there is (generally) a significant taste difference between whole grain mustard and regular yellow mustard. Texture plays a part of it, but it is secondary to the overall difference in flavor.

I say generally because there's a lot of variation in each, and some extremes of one type may approach some extremes of the other type in flavor.

Yellow mustard is usually very mild, and vinegar plays a very strong role in the flavor.

Whole grain mustard is usually stronger than yellow, and the vinegar flavor is much less pronounced, if not altogether absent.

In many situations, one type can usually be substituted for the other, although it is a matter of personal preference, and many people will argue that some uses of mustard are appropriate for one type, but not the other (for example, I would never use yellow mustard as a condiment for cheese and crackers, but I would use a whole grain mustard for some types of cheese).