"Flex" or derated takeoff thrust can be anywhere from 5 to 25% below max takeoff thrust, depending on the airplane, engine, and autothrottle computer implementation. It is calculated so that the minimum thrust is used to satisfy the runway and climb limitations for the airport (but NOT necessarily SID climb restrictions). Full rated takeoff thrust is seldom used, but exercised once or twice a month.
A common implementation is to use 2 fixed derates (e.g., -10% and -20%), with the option to use a calculated "flex" or "assumed temperature reduced thrust" instead. The "flex" or reduced thrust can be anything down to the max allowable reduction (e.g., -25%).
Climb thrust is usually a bit lower than the derated takeoff thrust. However, when using a large T/O reduction or derate, the climb thrust reduction is often based on full T/O thrust, which then may make climb thrust a bit MORE than T/O thrust.