Some points.
Having achieved a certain age I find that I am taller in the morning than in the evening.
I think that this is due to my spine (or at least the discs) extending in bed then compressing during the day.
Logically this means I am thinner first thing in the morning and my torso expands during the day as my spine shrinks.
So my waist measurement will change during the day.
Motto: measure in the morning
Another guideline is that your waist should not be more than half your height.
Allegedly the best guideline is the ratio between your waist and your hips, for example.
<http://www.msnbc.msn.com/id/14483512/ns/health-fitness/>
So - at the moment my waist measures 37 inches.
Can't find the "absolute figure" for men, but think it is around 36"
I am 6 foot (in the mornings) so half my height is 36"
Hip measurement is 39" so on the wait/hip ratio I am less than 1 so O.K.
One out of three can't be too bad - hope that it is the most accurate.
Cheers
LGC