What's the best way to get an eleastic waistband the right size?

I'm making a pair of shorts for someone far away and I was wondering how I
should measure the elastic for the waistband to make them fit just right?
If the person has a size 32" waist, should the elastic be just a bit smaller
than that or do you measure exactly the right size? Or allow a bit extra?
Advice, please.

Giselle (could never get this right on myself so heaven knows how I'm going
to manage it for someone not here)