The Cap Height of the sleeve is from the shoulder tip to the underarm line. The underarm line is about 1.5cm below the armpit.
See below for the examples of the Cap Height for a person's body, and the Cap Height on a block or pattern.
Note that the Cap Height on patterns change depending on the sleeve style; e.g. a flared sleeve will have a lower Cap Height that a Fitted Sleeve.
Example: Body Cap Height
The Cap Height of a person's body is from the shoulder tip to the underarm line. As the sleeve has to have some ease, the underarm line should be about .63-inch (1.5cm) below the armpit.
Example: Cap Height on a Pattern
This image shows a Fitted Sleeve Block (at the top), which has a Cap Height of 6 inches.
The block is used to make a pattern (at the bottom; the pattern isn't finalised). The sleeve is cut and spread to add fullness. This results in the Cap Height being reduced to 4.25 inches. (The bicep is also now larger).