I looked up some rates for electric and Heating Oil prices in MA, and your math looks right. I do believe you can graph the performance as a straight line between 17°F and 47°F and get a reasonable performance estimate. While in heating mode, your outside coils operate at a lower-than-ambient temperature, and they frost up. They'll frost up more with more moisture in the air, so the graph will not be 100% exact, but it should be a good guide.
By the way, I don't think this is a typical conclusion for other regions. MA heating oil costs about $4.688 per gallon, while PA heating oil costs $4.437 per gallon, so it's less than 6% more expensive in MA. But MA electric rates are about 30 cents/KWH to PA's 14.37 cents/KWH, so electric there is more than 108% more expensive than in PA.
But there is a second calculation you need to be aware of. Heat pumps are awesome because they use the energy in electricity to pull more energy out of the outside air and put it in your house. You can see this in the numbers you provided. The 22,200 Btu/h that this unit provides at 17°F, takes only 3.340 kWh with a heat pump, but would take 6.506 kWh with resistance heat. But 22,200 Btu/h is far short of your furnace's 94,200 Btu/h. Unless your furnace is almost criminally oversized, there will be an outside temperature where your heat pump cannot keep up with the heat loss through your walls and ceiling. What happens at that point is your inside temperature starts to drop below your thermostat's setpoint. Some inside units have built-in resistance heat that takes over if the temperature drops more than 2 degrees below the setpoint. But resistance heat is much less efficient. Each additional 10,000 BTU/h of resistance heat would require about 3 kWh in electricity.
Heat loss is generally proportional to the temperature difference, so if your existing furnace was designed to keep your house at 75°F on a 7.7°F day (a 67 degree rise), then a heat pump less than a quarter of the size would only provide a 16° rise, and would be ineffective in keeping your house from freezing on such a day. Looking at these numbers it seems to me, the advice to only use it above 40°F is because it may not keep up at lower temperatures.
If your units have no backup resistance heat, or if you can turn that feature off, you could experiment and see at what temperature the unit stops keeping up. You could also set the thermostat for the heat pump a few degrees above the thermostat for the oil furnace, in order to have an automatic backup should the heat pump be unable to keep up.
