How much extract?

I brewed a Honey Porter today. It is one of my favorite beers and I keep it on tap. I would like to try a twist on it and decided to add walnut extract during the secondary. I am not sure how much to add though. I thought about 1 oz for 5 gal. Is that too much, not enough? My mash was a little high at 157 but my gravity came out to 1.054 so I hit pretty close.

My recipe is:

Honey Walnut Porter

Grain Bill - 11.5#

Original Gravity (OG): 1.056 (°P): 13.8
Colour (SRM): 29.9 (EBC): 58.9
Bitterness (IBU): 65.6 (Average)

Grain Bill - 11.5#
63.83% American 2-Row
21.28% Rye Malt
4.26% Chocolate
4.26% Chocolate Rye
4.26% Crystal 120
2.13% Honey

1 oz Magnum (12.1% Alpha) @ 65 Minutes (First Wort)
.75 ox Cascade (5.4% Alpha) @ 20 Minutes (Boil)
.75 oz Willamette (4.8% Alpha) @ 10 Minutes (Boil)

Single step Infusion at 157°F for 65 Minutes. Boil for 65 Minutes

Fermented at 68°F with Danstar Nottingham

Notes: Add Walnut Extract During Secondary - 1 oz?

Thanks for your help.

I've never done this but,
you might want to try adding it to the keg.
Fill a glass with beer and dose with extract till it tastes good.
Then scale up so you'll know how much to add to the keg.