A final salary is the last salary you had working within a particular company. Employers often use this to determine what your salary expectations are likely to be if they offer you a post. If you are unsure or would prefer not to say (because perhaps you would accept a much lower or much higher salary than in your previous job) it would not usually look negative if you leave this blank.
Personally I would be very reluctant to leave it blank. It may indicate that you are looking to have a huge jump in salary. This may show a potential employer that money is more important to you than the actual job and I am not sure that is something they would want to see.
I would therefore be honest, but add in any bonuses, overtime etc as well.