This works really well on the A320 and A321.
Get airborne and at accel alt dial up 340kts.
Keep 340kts until the mach number is what you want to cruise at.
Swap to Mach on the FCU
When IAS is the same as in the climb perf page, go back to managed speed.
Reverse on descent
That's my relationship between IAS and Mach and it works just fine
