The picture below shows the use of a fillet weld gage. If a weld is convex, you only need to use the end of the gage shown under convex. Be sure to measure both legs.

Use the other for concave.

The welds are sized by the leg dimensions. The convex side measures the leg. The concave gage measure the throat. Both welds below are 1/4" fillet welds. One is convex, one is concave. The effective throats are the same.

The lines shown on the gage are a reference for the leg dimension whether the weld be convex or concave. The upper edge of the gage is the only part used for measuring the leg size of a convex weld. The lower edge is LARGER than the actual weld size for that gage. The vertical line can be used as a visual reference OR turn the gage around.

Have a nice day