You might like this technique that I use for setting tempo to something I've been practicing and am ready to record:
- Disable Stop at Project End in Track View Options if it isn't already so you can run the transport in an empty project.
- Start playback with no click sounding, count off 2-4 measures in your head, and stop the transport on what should be the next downbeat.
- Shift+M to open Set Measure/Beat At Now, and enter that measure and beat (e.g. measure 5, beat 1).
- SONAR will reset the initial tempo to make the specified beat fall on the absolute Now time where you stopped the transport.
- Note that Set Measure will also insert a tempo change of the same value at the time you set. If you're going to be changing the initial tempo later, you'll want to delete that one.