Speedbird,
You are correct that Tflex cannot be higher by more than 25% of OAT. Basically, the requirements for a flexible take off are :
1. Tflex is > OAT
2. Tflex is > Tref
3. Tflex is < or = Tmaxflex
For A320 only : Tref = ISA+30, Tmax = ISA+40, Tflex = ISA+55 (Tref and Tflex will vary accordingly for A319 and A321).
Simple example : airfield elevation 3000 ft.
--> 2 deg C per 1000 ft lapse rate = 3 x 2 = 6 degrees C
--> 15 degrees standard temp at sea level = 15 - 6 = 9 degrees C
--> Tflex from above = 55 + 9 = 64 degrees C, this means that your flex temp for take off should not exceed 64 degrees (25% of rated thrust or Tref). This is the last row of flexible temp you will see on your RTOW chart corresponding to that particular ATOW.
Tmax for the above example would be = 3 x 2 = 6 degress C
--> 40 + 6 = 46 degrees ( at 46 degrees OAT you cannot take off )
I hope this helps
Cheers