Very common and not a Tiara specific issue. If you use a photo tach to calibrate the difference you can just adjust mentally. Where it matters is those engines should hit 100-150 RPM above rated WOT. It really matters and accuracy of your tach is crucial. For Cummins that would be 2700+ RPM.
I installed Aetna tach on my prior boat and loved it. Some don't like the look of the digital display, I did. As mentioned above, once you photo tach you can make a note of the delta, even make up an index card chart of gauge vs actual..
Good move on the photo-tach. That's what I did on my prior boat and just made note of the true vs. gauge delta. But then a couple years later I built a custom dash with all new instrumentation so bit the bullet and installed an Aetna. Single screw so half the cost..