The regular weekly working hours in flexitime must average at 40 hours within each period of four months.  The excess (so-called "plus hours") at the end of each period can be no more than 60 hours and the deficit (so-called "minus hours") cannot be more than 20 hours. During the fourth month period, the balance of working hours can temporarily higher than 60 hours as long as the balance at the end of the period does not exceed the 60 hours limit. You can decrease the working hour balance by reducing your daily working time or having days off. 

Working Hours Act allows you also to divide your working day into two parts. You can for example agree that felxitime is in use between 8 pm and 10 pm instead of immediately following the end of your fixed working hours, provided that it won't constitute a break to your fixed working hours. Fixed working hours refer to the time when you are obliged to be at work.  You can agree with your employer on changes to the existing flexitime arrangement. Issues  such as the amount of excess and deficit, maximum balance and four month adjustment periods are regulated by national collective agreements.

Agree at least on fixed working hours, which specify the time when you have to be at work. Agree also on how the flexitime period is determined, ie. what are the time periods during which you come to work or leave from work. You should also agree on the amount of excess (plus) and deficit (minus) hours and on the placement of rest periods.