"The QNH being the regional pressure in your current area based on your current height above mean sea level."
No, you got this wrong. The term height and QNH shouldn't be used together. Altitude is the correct term.
Height is read using QFE.
What you describe is the Regional QNH, not the local /airfield QNH.
Regional QNH is forecast and published in advance and is used for en-route terrain clearance, whilst local/airfield QNH is measured at the time and is set prior to making an approach.
No-one should try to make an instrument approach on regional QNH.