During my training, I learned bitten nails are one of the most difficult to work with. You have to be super thorough with your prep. I usually sculpt on bitten nails, but if you must tip, shallow or well-less tips are probably best because bitten nails don't have a whole lot of surface to work with. For biters, its best to keep the length short in the beginning, probably just long enough to cover the finger tip... a functional length. Because their nails are shorter than normal, its not a good idea to add on too much length. You have to be really careful not to get any product on cuticle or sidewalls because biters are usually pickers as well. If there's anything that shouldn't be there, they'll be tempted to pick at it. I use Young Nails syngery gel. If I have to perform a gel enhancement service for a nail biter (I prefer l&p for nail biters), I protein bond (primer) twice. Reccomend they use their cuticle oil regularly, and keep their fingers out their mouth. And I have them rebook sooner than 2 weeks for rebalance/backfill just to fix any lifting or regrowth ASAP.