Extract quantity

I am going to brew a banana bread stout in a week or so. I was planning on using a banana concentrate (extract) and/or dehydrated banana chips as well as some spices. I asked a clerk at my local home brew shop about when I should make the additions and he suggested adding a 1/4 during the first 20 mins. of the boil and then adding the rest in the final 10 mins. Firstly, do you agree with his suggestions? Secondly, how much banana extract should I add? I am using a 5 gallon brewing kit. I don't want the stout to be overwhelmingly banana flavored but I also don't want to go too light. One idea I had was to start with a small to moderate amount of extract and then add dehydrated banana chips in the secondary fermentation if the flavor needs some extra help.


Any and all suggestions very appreciated. Thanks!