A320 Flex-Displaced threshold
 

An experienced pilot told me that for every 300 meters displaced threshold deduct 3 degrees flex,when I asked for a reference the answer was that's by experience any idea.

Never heard of that.
You should always use the correct performance for the runway intersection you are using.

Use the chart which covers the intersection you are taking off from to be on the safe and legal side.

On a 4000m runway a 300m displaced threshold wouldn't have any effect.
2500m, hot day and heavy aircraft 300m would have a considerable effect.

Good morning,

It will probably work indeed. But your 'experienced' collegue is missing the point entirely. We don't do performance calcs to satisfy ourselves on basis of experience.
It's is the same as saying that 140, 140, 145, flaps2, flex45 will work anyday. It will probably work 90% of the time......

The reason we do performance calculations is to satisfy either JAR25, FAR25, CS-25 etc. requirements not to make ourselves happy!

There is no reference on flex temp adjustments for rwy length in any Airbus documentation. Airbus only speaks about flex temp corrections for QNH/bleed corrections.
A flex temp correction only makes sense if the new condition (i.e. QNH, bleed req.) only penalizes effective engine thrust. A last minute reduced runway length influences a dozen of parameters and cannot be mitigated by increasing engine thrust only in ALL conditions.

Regards