NR Adjustment
The Arriel 1 has a "design feature" that causes the RPM to vary with fuel temperature.
As the RPM drifts off all the vibration absorbers fitted to the head and under the floor then go out of tune.
As stated the NR Tachometer cannot be interpreted accurately enough and can be adjusted to suit.
Various methods can be used to use to measure the RPM via a simple break out harness. For example the MicroVib balancer has a page where you can input the frequency value that equates to 100% and/or an RPM value.
It is important that it is checked in flight at or about MCP in the cruise. The RPM is different between models.
Keeping the RPM in the correct range is probably the most important thing you can do to keep your aircraft smooth and should be checked regularly and before any other adjustments are made.
355 F's with pneumatic governor controls can be anyones guess as the RPM drifts with barometric conditions although there are aftermarket kits to improve this.