Lemon Extract has a bitter unreal taste to it and I would use a real lemon and squeeze out the amount you need. If you don't have lemons you are better off to use lemon juice in the bottle. If you can't get out and get either one then you could get away with Lemon Extract, but read directions carefully and don't put too much in as it's more concentrated.
I'm not an expert, but you would need alot of lemon juice.