Almost forgot. I was told to assume that prime powder, when it burns, sends sparks and flame up and away from what ever surface it is in. Which is why you want the touch hole slightly above the bottom of the pan. may an 1/8 inch to 3/16. The shape of some traditional pans seems to have an effect on the speed of ignition. Some are square depressions, some are cylindrical and some are slightly egg shaped. When the pan area immediately next to the touch hole is round, the walls of the pan depression seem to direcct the fire and sparks back toward the touch hole. Think of it a bit like reflecting light back off a mirror. if the mirror if flat the light just goes back in one direction. But is the mirror is concave like a bowl, the light is concentrated into one area. you want the pan surfaces to reflect/direct the spark and flame to be more concentrated at the touch hole. For that reason, I prefer the cylindrical and egg shaped pans rather than the square ones.
If you need to resort to grinding some away as Berks indicates, keep the shape of the pan area next to the touch hole in mind and try to keep the sides where it meets the barrel flat in a rounded or slghtly oval type shape.
I would caution you against just grinding an small area just next to the barrel flat, or you will create a funnel to channel the prime powder down between the barrel flat and the pan. You still need the powder spread slightly across the bottom of the pan to catch as many sparks as possible. If the powder is just concentrated in small hole you ground, there is less chance of sparks hitting it.