I presume you are referring to using
Flying Blue miles.
It used to be that the Miles Finder showed how many miles were required for an upgrade - but I guess with the recent changes that has ended. The Miles Finder now just says:
** When you book a regular ticket (i.e. not an award ticket), certain conditions permit you to use your Award Miles to upgrade to Business Class. For more information, contact the Flying Blue Service Center.
First of all, are you even sure that you have booked your AF flight in an upgradeable class?
See the following thread which lists the fare classes which are upgradeable, and also has a link to a spreadsheet showing the upgrade amounts (I can't promise that this information is still up-to-date).
http://www.flyertalk.com/forum/flyin...-upgrades.html
You could always ring Flying Blue and request this information.
Note, also, that if you are hoping to upgrade a Business Class fare to First, you need to have Flying Blue Elite or Elite Plus status.