Sorry I'm too late to help you with your Xmas cookies, but and eighth sounds like more than enough if you're only using a quarter cup of oil and only making a few cookies. There doesn't seem to be any good formula for how much to use, and it really comes down to personal preference. On the few occasions I've used it for baking, I just do a whole pan of brownies or cookies, and do the math on what is probably a good dose for me. I made them too strong the first time, so that might be more likely to be a problem than too little. Happy baking!
