A: The vertical scale on a P-h diagram is logarithmic, meaning the spacing compresses as pressure increases — so the true midpoint between 100 and 200 psia is higher than the visual halfway point. If you're in that wide span, your best precision tool is the saturated properties table: look up the saturation temperature at your pressure, then come back to the chart horizontally to find the exact point. The difference might literally be the thickness of a pencil line, but in a tight range like 100–200 psia, using the table to anchor the point is worth the extra step.