This will depend heavily on your department's guidelines for marking and your supervisor and second assessor, some will be a lot more generous than others and so it's quite difficult to give any good advice on this.
I would say given the amount of supervision and feedback you should have gotten for a final year project, you should have a pretty good idea already what kind of mark you will get (unless your second assessor is rather unkind and your supervisor is the opposite). If you don't already have a good idea then you're probably not going to do that well, emphasis on probably.
For word count specifically, I would say don't go below, they should give you a range that it should be in and if it's not in that range then you may have problems. It's ok to go above if it's a high word count dissertation, say 30+ pages is expected, but if it's a lower count one e.g. 10 pages then I would try to avoid going over and you should just stick a lot in the appendix instead.