From one of the largest Q400 operator:
2.12.8 Final Approach Speed
The final approach speed (VAPP) is the target speed for the final approach once the
landing configuration has been established.
Typical VREF values are 100 to 130 kts; so VREF provides a margin of between 20 to
30 kts above the stalling speed.
During a moderately turbulent approach, variations in IAS of up to 20 kts might occur.
A reduction in speed could bring the aircraft close to the stalling speed. In turbulent air,
it is acceptable to target a speed above VREF on the final approach in order to maintain
an adequate margin over the stall.
VAPP is calculated by applying a gust factor to VREF.
VAPP = VREF + gust factor.
The gust factor is half the variation in wind speed or 10 kts whichever is the lower. It
does not depend on the direction of the wind and is not bugged. For example, if the
wind is reported as 20 kts gusting 28, the gust factor is 4 kts.
Note: If VREF is already enhanced for any reason, then VAPP = VREF and the
approach is flown “on the bug”.
2.12.9 Threshold Speed
The threshold speed is the speed at which the aircraft should cross the threshold. The
basic threshold speed is VREF.
The aim is always to reduce the final approach speed such that the threshold is crossed
at VREF. If the threshold is crossed at speeds above VREF, the safety margins on
landing distance may not be met.
2.12.10 Touchdown Speed
The touchdown speed is the speed at which the aircraft touches down. If the threshold
is crossed at VREF and if the normal flare manoeuvre is performed, the touchdown
speed is normally 5 to 6 kts below VREF.
Hope this helps.