What should my BMI be for a BBL?

Thank yoyu for your question. Patients should have a BMI lower than 30 for best results. Patients with a Brazilian butt lift can look forward to a double benefit: reducing areas of unwanted fat while gaining a perkier, rounded posterior. Plastic surgeons plan each procedure according to the individual needs of their patients, harvesting fat from target areas of the torso, thighs or knees and transferring it to create smooth, attractive contours optimized for each person. Many patients welcome the idea that their own tissue can be used to enhance their shape. The amount of fat transferred from love handles or flanks depends on the quantity available and the extent of buttock augmentation you desire.  That said, You may not be a good candidate if you’re very slim with little fat to spare. However it's best to get a consultation with a board cetified plastic surgeon. 

Hello dear!
Thanks for the question and provided information as well.
Your BMI (24) is optimal for surgery. In general, you need to be physically healthy and at a stable weight, have realistic expectations, be a non-smoker and your BMI needs to be 30 or less. 
l recommend you to make an appointment with a board certified plastic surgeon to talk about your goals and anything you need to know.
Good luck :)