Now when writing or recording an idea we can easily test if we've done our equation correctly.
Start your recording at 90bpm and play the riff for a minute or more.
Figure out where and when your riff turns around (in this case it’s after 3 beats).
So I've now figured out that my riff turns around after 3 beats and I'll go ahead and call that 3/4.
Now I let it repeat 4 times just to be even and I'll mark the downbeat of the 5th measure.
I'll now switch the tempo to 120 on that downbeat and press play. If this is done correctly you will get the exact results that you here in the recording.