Most of the oldtimers use 1/4 teaspoon of 90,000 HU cayenne pepper. The HU stands for heat units. The regular grocery store stuff is typically 30,000 HU. You can find the 90,000 HU stuff in the bulk food section at Whole Foods or on the internet.
To much cayenne pepper can cause external itching with some people. So if you start itching like on the back of your hands for instance, it might be the cayenne.