Why make it so hard ?
The sensor is ratiometric, that means 5V can be used in the calculation, even if the actual 5V voltage varies a little.
The sensor is input 0...100kPa which corresponds to an output of 0.2 to 4.7V.
That means the range of 100kPa corresponds to a range of 4.5V (with 0.2V offset).
I strongly prefer to calculate the voltage at the pin, to be able to write it to the serial port and check it with a multimeter.
int rawADC = analogRead(A0);
float voltage = (float) rawADC / 1024.0 * 5.0; // 5.0 is always good, the sensor is ratiometric
// correct offset, divide by voltage range, multiply with pressure range.
float kpa = ((voltage - 0.2) / 4.5) * 100.0;
If you want to add a filter, keep it seperated from this calculation. Use the resulting float 'kpa' variable as input for a filter.
We all use kPa of course. Have you been watching too much Cody's Lab ? Making a Barometer With King of Random - YouTube
To get mmHg:
// According to: https://en.wikipedia.org/wiki/Millimeter_of_mercury
// 1 mmHg = 133.322387415 Pascals, which is 0.133322387415 kPa.
float mmhg = kpa / 0.133322387415;