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)
Then select Measures beats and enter the length of the clip in bars/beats.
Take a note of the new playback rate (0.666667 in this example)
Now use a calculator or Python. You want to divide \[\frac{\mathrm{current\ tempo}}{\mathrm{playback\ rate}}\]
So in this case \[\frac{120.0\mathrm{bpm}}{0.666667}=179.99991\mathrm{bpm}\simeq 180\mathrm{bpm}\]