I use this website to figure out what the difference between Absolute and Relative should be.
https://keisan.casio.com/exec/system/1224575267Unfortunately this site uses metric units. But that is actually a good thing. Because with Ambient you can be more precises if you calibrate pressure using hPA. Then after all your calibration of Absolute and Relative is done then you can tell the console to switch to inHg if you prefer and the calibration sticks.
So to enter in numbers to the above website you'll need to convert your elevation in feet to meters. Google does that in the search bar, just enter 348 ft to m and serach for Google to give you the results. For example if you are at 348 feet of elevation that is 106.07 meters. So you enter in 106.07 meters for your elevation. But lets round to the nearest whole meters as that is better to match online services. So you use 106 meters. Then you enter in 1013.25 for the atmospheric pressure always. It doesn't matter what your actual pressure is at that moment. At this point this is still Step #1. We just want the offset that corresponds to your elevation and that is constant. So we get the best results if we use standard pressure which is 1013.25 hPa. Also leave the temperature at 15 deg C. That is standard temperature and we keep that number too. Then you press the Execute button.
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As you can see the results yield 1,026.05 hPa. You might say okay what does that tell me? It tells you how much the offset is for your elevation between Absolute and Relative Pressure. So you take the 1,026.05 and you subtract 1013.25 to get the offset.
1,026.05 hPa - 1013.25 hPa = 12.8 hPa
Now we know that in your console Absolute Pressure will always differ from Relative pressure by 12.8 hPa. So if the airport says the pressure is 1017.21 then you know that your absolute pressure should be 1004.41 hPa. I just subtracted 12.8 from 1017.21 to get the Absolute pressure.
What we have learned from this is that at elevation 348 ft your Absolute pressure will differ from Relative pressure by exactly 12.8 hPa. By telling your Ambient console that the offset is 12.8 between Absolute and Relative what you have actually done is give your station your elevation in an indirect way. It doesn't use your elevation...but is just keeps Absolute and Relative different by those amounts.
If you want to convert from hPa to inHg or vise versa then use this website:
https://www.convertunits.com/from/hpa/to/inhg12.8 hpa is about 0.378 inHg. So your Absolute and Relative would differ by that amount.