How can one set a default margin value of one half inch for printing files using OpenOffice org 3?

It is one eighth.

1.5

"one half-inch" - Grammatically, wrong. One-half anything is correct, but usually 1/2" or even 0.5 inches or 0.5" are far more meaningful to most readers. If we customarily used half-inch as a unit...

Everybody's hall is different so you have to measure your hall

There are exactly 1.5 inches in one and one half inches.