Anywhere 1/2 Seat is shown for a Trouser, this means the Finished Seat divided by two. If you provide your client's skin seat measurement divided by two, the trousers will fit too tightly over the seat.
A recommended value to add to your client's seat measurement is 2 inches (5 cm). As you'll see in the example below, the client's skin seat measurement is 40. 40 + 2 = 42. 42 divided by 2 = 21. So in this case, the 1/2 Seat is 21.
If the client carries large items in his pockets or just prefers more room in general you can add 2 1/2 or 3" to his skin seat measurement.