how far away from the enclosure wall should my port be?

yo,
i have made sevral posts in the past few weeks and all have revieled very useful information......

well here is my question for the day......

i am constructing my first subwoofer.....
it is a vented enclosure.....the vent being 3inches in diamater......
the port has to be farly long to tune the box to 25hz........
how much room should i have between the end of the port and the enclosure wall?

should i use a elbo?



thanks,
slice:cool: