To give you a quick understanding of how to know the best ohm coil FOR YOU, on your Provari, simply download the ohms law app onto your smart phone. It asks you for two values out of 4 fields (volts, amps, ohms, and watts), and populates the rest for you. So lets see what happens if we want to vape that 1.8 coil on the provari with the voltage set to 3.7.
Voltage = 3.7
Ohms = 1.8
Once those values are entered, the Amp field and Watts field self populate with the following values:
Amps: 2.055556
Watts: 7.605556
Now the Amp limit for the Provari is 5, and the Watt limit is 15. So the above shows your good to go. But maybe you want to bump up the voltage a bit. Lets say you try to go up to 5.2 voltage with that 1.8ohm coil. Once the voltage value of 5.2 is entered into the volts field, and 1.8 is entered into the ohms field, you get the following values for amps and watts:
Amps: 2.888889
Watts: 15.022222
Your fine on the Amps as Provari's limit is 5, but Provari's limit on Watts is 15, so you will get the good ol' "E4" error when you try to vape at this voltage. But bump down to 5.1 volts on the provari and your good to go.
The reason it was suggested to go to a 2.2, is because you can have a little more free range on your Provari's wattage selection and go up a bit higher, likely up to a 5.7 or so, and Provari goes to 6. So you will have a bit more freedom until you get an error message. But does that necessarily mean a 1.8 coil vaped at a 5.1 is not as good as a 2.2 at 5.7? No! It does not mean this but you are creating .3 more Watts or so with the 2.2 coil at 5.7 than the 1.8 ohm at 5.1. Probably not significant enough to see much of a difference.
Hopefully you understood all this and are not confused. Ask questions if you are because this is the key to great vaping and safety, especially when rebuilding.