A few thoughts that might help:
1. If the tempo of the clip is misdetected, SONAR will tend to insert superfluous "User" markers (stubby markers with solid diamond handles/heads) at quarter-note beat markers in the timeline where there are no actual audio transients. Usually there will be another tempo in the clip tempo map dropdown that's closer to the actual tempo that will eliminate most or all of these superfluous User markers when selected. then you can select any reamining User markers and delete them (you'll want to bind a key combo to Delete Marker to speed up the process, since each Delete command deletes only one). One way or the other, it's a good idea to get rid of those before doing anything.
2. In addition to adjusting the Threshold, you can enable or disable markers using the transient tool's context menu. Promoting a marker locks whatever that state is whether enabled or disabled so the Threshold setting no longer affects it. I like to get the clip in a state where all of the markers and only those markers I want are enabled, and then Promote them all.
3. For a drum clip that originated with a drum machine, it's likely that the tempo is fixed and consistent, in which case the best thing to do would be to find the average tempo of that clip, and set that one fixed tempo as the initial tempo in SONAR. I like to do that using Set Measure/Beat At Now instead of Audiosnap:
- Drag the clip to align the first transient to its corresponding beat in the timeline (e.g. could be a pick-up note on the last 8th of the first measure).
- If that time isn't 1:01:000, use Set Measure/Beat At Now (Shift+M) to "pin" that first beat so that becomes the reference point for determining tempo.
- Count out several measures listening to the clip set the Now time at a downbeat transient using Tab-to-transient, and use Shift+M again to set the correct measure and beat in the timeline to the absolute time of that event.
- SONAR will alter the tempo at the first point you pinned to make the specified timeline beat fall on that transient, and add a like tempo value to the beat you set as a reference for setting later beats if needed.
- If the clip was recorded to a click, that may be all you need to do; if not, you can set additional beats every few measures or every measure, or even within measures or beats (note that beats are divided into thousandths, not 960 ticks, so 02:480 is 02.500). With your drum machine clip, you're likely to find that the calculated tempo comes out very close to some whole number of beats per minute, and you can go back and set the project at that tempo.