What was the favored operating vacuum level (in inHg) established by the Great Western Railway (GWR)?

Answer

25 inHg.

While most British railways settled on 21 inches of mercury (inHg), the Great Western Railway (GWR) specifically favored a deeper vacuum of 25 inHg, which sometimes caused operational issues when meeting other company locomotives.

What was the favored operating vacuum level (in inHg) established by the Great Western Railway (GWR)?
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