1 teaspoon dry mustard = 1 tablespoon prepared mustard or ½ teaspoon mustard seeds

1 tablespoon prepared mustard can be substituted for 1 teaspoon dry mustard

The conversion rate between mustard powder and prepared mustard is about 3. This would mean that you must use 3 times as much real mustard to substitute the powder. IE, 5ml (1 tsp) powder = 15ml (1 tbsp) prepared mustard.

Donald Mustard is 5' 11 1/2".

1 teaspoon is 5gm. 1 tablespoon is 15 gm. So your mustard is more suited to 1 teaspoon.