The appropriate amount of carbonation for a porter

I will be bottling a london porter soon and I've been thinking about how I want it to turn out. I like my dark beers to be a little warmer when I drink them and with less carbonation, kind of like a cask conditioned ale. Every recipe, including the kit I have, says to put a 2/3rd cup of dextrose into the bottling bucket. I'm wondering if you all have any opinions about obtaining the appropriate level of carbonation for the porter style.

John Palmer's book has a great chart for helping determine carbonation, as follows:

How to Brew - By John Palmer - Priming Solutions

Use a straight edge to make a line from the temperature you want, over to the carb level you want (middle column.) Keep following it from there and it'll show you how much priming sugar to add for a 5 gallon batch.

It's also handy that there is a list of recommended ranges for various beer styles. (This chart is one of the handiest brewing tools I've come across in recent months.)