We base all of our text heights on point size, which is the same basically as the Leroy sizes. This way if the client(and some still do this) is addin stick up notes to the final polo, everything will match.
In engineering we mulyiplt the point/leroy height by the scale for the final size. EXAMPLE: 8 point text = Leroy 80 guide (L80) = 0.08" at a scale of 1"=30' would be 0.08 x 30= 2.4. A point is roughly 0.01" so L60=0.06", L80=0.08", L100=0.10"etc.