which db ? 10 or 12 ?
you should start just as it says in the plans (or somewhere on the site) with the insert a few inches above the port and the move it 1/4 inch until it sounds best. !!!never use lots of power until it's completely tuned, sealed  and the insert is fixed into place !!! . i had trouble with mine and i was very dissapointed at first.
what driver are you using ?
i have my db-12 tuned as low as possible and it sounds great like that.
the insert in my db-10 is just above the port. some say it sounds more 'natural' this way. with my current settings i get almost flat from 40 to 100. below 36 (?) the response drops. that drop is probably the woofer's fault (can't wait to change the cheapie  :D ). i'd say not to tune lower than half the port because port noise won't be the only problem.
most important : make sure you spend lots of time with tuning and listen to you're favorite music cds to tell when it's right. you can't find two people with the same listening tastes, so tuning is very subjective

oh, make sure you round up the edges on the halfmoon. if you don't have a router don't worry, it's easy to round with some sandpaper and it's sure worth the effort