Hop Tea? -> Secondary

I am trying to increase the IBUs last minute on an low IBU IPA.
I'm Looking for some advice/input on how to go about adding a hop tea.

I was thinking to just boil .5 oz of hops(12% AA) for 30 min in about 20 oz of water and then strain & cool => add to the secondary (5 gal).
Problem is I dont know if it wil be too much or too little . I'm aiming for 15 IBU increase. Any suggestions on hitting the mark?

I've only used hop tea by adding, stirring (gently), adding, until I had it right. I haven't found any charts that deal with pure water. But http://hbd.org/cgi-bin/recipator/recipator predicts boiling 0.5 oz. of 12% AA hops for 15 minutes would be 16 IBU once you dilute to 5 gallons.

I got the same numbers with brewsmith but wondered if the small boil volume would somehow decrease bittering extraction .

Looks like .16 gal (20 oz) of pure water will make a 502 IBU tea. Diluted to 5 gal will come out to a 16 IBU increase, like you said david_42 .

So I went for it with .5 oz for 15 min boil in 22 oz water. I'll let it sit in 2ndary for 2 weeks with some dry hop.

Appreciate the help