A320 Unreliable airspeed and trouble shooting

[CMpilot1]

In the simulator I have noticed that in the process of isolating the affected ADR by using the QRH pitch and thrust tables for a given FL and weight, the resultant speed indicated in the QRH table and the speed indicated by the reliable ADR does not match exactly. Sometimes it's way-off by at least 10 knots or greater. Is there a tolerance allowed? And how will you identify the reliable ADR if the resultant speed on PFD after setting the pitch and thrust does not match the QRH speed?

[Check Airman]

I’m happy with 10kt. In the grand scheme of things, it’s pretty good, I think. If you were on final and 10kt too slow, the wings will still be safely attached to the sky.

[vilas]

In the simulator I have noticed that in the process of isolating the affected ADR by using the QRH pitch and thrust tables for a given FL and weight, the resultant speed indicated in the QRH table and the speed indicated by the reliable ADR does not match exactly. Sometimes it's way-off by at least 10 knots or greater. Is there a tolerance allowed? And how will you identify the reliable ADR if the resultant speed on PFD after setting the pitch and thrust does not match the QRH speed?
Pitch thrust tables are approximate so 10kts difference is OK. If one ADR is unaffected then the speed given by that will be more accurate than the table. Now new aircraft have the Digital Back Up Speed or DBUS, which you are supposed to use in case of all ADR unreliable. It is a calculated speed without pitot/static input using AoA, load factor, Wt. and GPS data etc. That is also accurate within ten kts.