I suggest you use a related table which has a record for each day of sick leave that a person takes. If a person takes sick leave you record the date and the employee number In the record.
I would recommend you first take the Lynda.com FileMaker database class so you have basic database fundamentals. Otherwise, design will always seem mysterious instead of matter of fact simple.
Regarding figuring out how to do sick leave and such -- make sure you can figure it out on paper first. Run through all the possible scenarios. THEN (and only then) go to the computer and implement it.
Altogether FileMaker isn't (doesn't have) a real programming language, the same rules apply: "programming" should be the easy part if you've done your homework and analysis up front. If you can't figure it out on paper, then you need to go back to the stakeholders and get better requirements, usually.
Think about reports too. What kind of data is needed for reporting? That will help drive the data design.
HOPE THIS HELPS.