If you start with a tempo of 120bpm, and load in a clip of a different tempo,
cut out a clip of exactly 8 bars (use the kick if you can) by splitting at
the start and end (take a copy of the clip to preserve the original). 
Open item properties (F2), and click Set (Playback rate)

![]{centre shadow}(SetPlaybackRate.png) 

Then select Measures beats and enter the length of the clip in bars/beats.

![]{centre shadow}(SetMeasuresBeats1.png) 

Take a note of the new playback rate (0.666667 in this example)

![]{centre shadow}(NewPlaybackRate.png)

Now use a calculator or Python.
You want to divide
\[\frac{\mathrm{current\ tempo}}{\mathrm{playback\ rate}}\]

So in this case
\[\frac{120.0\mathrm{bpm}}{0.666667}=179.99991\mathrm{bpm}\simeq 180\mathrm{bpm}\]