Here is how I measure my stoma and make a near-perfect template: When you take home a salad from a restaurant, ask for the dressing in one of those little sauce cups with a lid. Wash and dry it, and then use the inverted cup as an overlay over your stoma. Trace the outline on the bottom of the cup. Then cut the hole in the cup and use this as your template for tracing the outline on your wafer. You will get an accurate size and shape. Very easy!