Is it suppose to be Equal percentage?
Offset butterfly valves address butterfly valve seat and stability issues.  The equal percentage characteristic is easily machined into a globe valve plug or cage.  A butterfly is more linear through the middle travel, likely quick opening at the beginning and perhaps the end of travel.  You may be able to characterize a valve with a smart electronic positioner; or cams on a mechanical positioner.  Fisher provides lots of capacity data.  Use the Fisher butterfly valve data and draw some curves.
The curves of triple offset valves usually may be considered about halfway between equal percentage and square root characteristics... resulting in a linear behaviour (and therefore a good rangeability) far from the complete opening and closing positions, i.e. approx. in the 20°÷70° or 20%÷70% range.
For more specific information, of course, refer to the Manufacturer.

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