Overall, that seems like a solid plan. A pound of pecans seems reasonable enough and if you end up with excess extract you can always save it for a later batch or anything else that could use some pecan flavor.
In terms of determining an appropriate amount to add to your beer, the best method is probably to measure out small quantities and try small mixes until you find a ratio that fits the level of flavor you want. If you have a beer that's similar to the one you're making on hand you can use that as a base beer, or you can take a sample of the beer you'll actually be adding it to and try it that way. Using a small syringe to measure out 1/2/3 mL of extract at a time and mixing that with static amounts of beer is probably the most accurate method in terms of being able to scale it up to the full batch size. But as long as you have some repeatable way of measuring small amounts of the extract you can then do the math to figure out how much you need to add to the whole batch to achieve the effect you're looking for.
It might be a little tedious but can eliminate a lot of the guess-work, especially if it's easy enough for you to make a lot of extract. Especially since someone else's extract might not have the same strength/quality of flavor as yours and the amount they like might not be the same amount you'll like.