I had my gallbladder taken out when it was at 17%. I am not sure what the threshold is for removal, but after I felt SO much better. I wonder if your is diseased or just not functioning well. I know that they told me that 30-35% was normal. I wi dear if a different doctor would be willing to remove it, if that is what you want.
Meanwhile, if Dr says it's not bad enough to remove, then I would eat very carefully. Avoiding fats helped me before my surgery. I went oN an elimination diet and found out what foods I could eat without pain. I feel for you, gallbladder pain is pretty intense.