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[This snippet of information has been sitting in my email folder for a few years now, and whenever I need to calculate an accurate BlueScale value in a font, I have to go and find the message from Adobe font tester extradordinaire Ernie March in which the equation occured. So this wiki entry exists in part to make it easier for me to find the information when I need it, but I figured others might like to know how to correctly set the BluesSale value for a PS font (Type 1 or CFF OpenType.]
The BlueScale value can be expressed in a number of ways, and one sometimes sees it expressed in point size. In the following, the BlueScale is expressed 'as it is' (to use FontLab's terminology), i.e. as a positive real number value.
A correctly set BlueScale value is relative to the largest BlueZone (vertical alignment zone) in a given font, which in turn is relative to the UPM value. It should be noted that the default BlueScale value suggested by font tools usually presumes a UPM of 1000, so fonts with larger UPM values, e.g. 2048, will need to have the BlueScale recalculated. Similarly, if a font has a particularly large BlueZone, the BlueScale may need to be recalculated even for a font with a UPM of 1000.
A correct BlueScale value will equal to or less than 1/x, where x is the height of the largest BlueZone in the font.
So, for example, if the largest BlueZone in the font is 36 units, the BlueScale value should be equal to or less than 0.027777777777777777777777777777778. [In this case, I would typically set the value to 0.0276.]