Our 319's with IAE's are flat rated to ISA+40, max flex temp is ISA+65...
OP is talking about minimum Flex.
Have a loook at your fcom...and look. for MINIMUM ASS or FLEX temp.
The Fmc wil not allow you to enter a flex temp that would calculate a thrust setting higher than your max thrust rating(if you use derate,max thrust for your derate) for actual OAT and PA.
The engines are rated at Sea level and isa+15c, so at sea level your min flex should be about 30c.