what is the ideal ratio to mix mustard powder with flour?
If want to sell mustard, make mustard, not a stretched fake mustard. You would be cheating your customers and depending on your location the authorities might be after you, too.

The "sharpness" of a mustard is balanced by the ratios of different mustard seeds, white/yellow, brown or black and the temperatures used during preperation. Typically, vinager is added and sometimes other flavouring compounds like other herbs, spices or honey.