It is not quite clear. Can you kindly better describe the requirement?
A mockup is welcome.
Regards
It is not a good approach to previouly display the next booking number because, lets say, two receptionists create records at the very same time! It may through an exception on database once that field can not be duplicated. Actually we faced that twice on different customers years ago!
So you really need to let SugarCRM to auto generate this field as soon as its record is saved into database.
Regarding the issue on your instance, I'm afraid that your custom CreateView has not been properly implemented, which may been leading to a kind of looping.
Francesca Shiekh's suggestion certainly will work, not exactly as you would like to have implemented, but this is the best approach you may find.
Regards