When the boiling point at standard atmospheric pressure is known …
Place a ruler across the page such that it intersects line B at the normal boiling point of the substance. The ruler then intersects line A at the approximate boiling point of the substance under the reduced pressure indicated where the ruler intersects line P.
When the pressure is known, tilt the ruler so it crosses lines B and P at the known values. The boiling point under this reduced pressure can now be estimated from line A.
To estimate the pressure at which a substance will have a convenient boiling point, tilt the ruler so it crosses lines A and B at the appropriate temperatures. The pressure required can now be estimated from line P.
When the boiling point under reduced pressure is known …
Place a ruler across the page such that it intersects line A at the observed boiling point, then tilt the ruler so it intersects line P at the observed pressure. The boiling point of the substance under standard atmospheric pressure can now be estimated from line B.