This .NET class wraps the AcDbPlotSettings::stdScaleType ObjectARX class. 

The stdScaleType property stores the "standard" print scale selection, including scale-to-fit, for the plot settings object. It is not a number like customPrintScale, but is an enum indicating the desired print scale. AutoCAD uses this, along with other information like the plot area, to compute the customPrintScale when useStandardScale is true. While the stdScaleType property always has a value, it may or may not be reflected in the customPrintScale, depending on the value of the useStandardScale property.

Public Enum StandardScaleType
  CustomScale = 1
  Scale100To1 = &H12
  Scale10To1 = &H11
  Scale1ftAnd1ft = &H22
  Scale1inAnd1ft = 30
  Scale1To1 = 2
  Scale1To10 = 7
  Scale1To100 = 13
  Scale1To128inAnd1ft = &H13
  Scale1To16 = 8
  Scale1To16inchAnd1ft = &H16
  Scale1To2 = 3
  Scale1To20 = 9
  Scale1To2inchAnd1ft = &H1c
  Scale1To30 = 10
  Scale1To32inchAnd1ft = &H15
  Scale1To4 = 4
  Scale1To40 = 11
  Scale1To4inchAnd1ft = &H1a
  Scale1To5 = 5
  Scale1To50 = 12
  Scale1To64inchAnd1ft = 20
  Scale1To8 = 6
  Scale1To8inchAnd1ft = &H18
  Scale2To1 = 14
  Scale3inAnd1ft = &H20
  Scale3To16inchAnd1ft = &H19
  Scale3To32inchAnd1ft = &H17
  Scale3To4inchAnd1ft = &H1d
  Scale3To8inchAnd1ft = &H1b
  Scale4To1 = 15
  Scale6inAnd1ft = &H21
  Scale8To1 = &H10
  ScaleToFit = 0
End Enum

public enum StandardScaleType {
  CustomScale = 1,
  Scale100To1 = 0x12,
  Scale10To1 = 0x11,
  Scale1ftAnd1ft = 0x22,
  Scale1inAnd1ft = 30,
  Scale1To1 = 2,
  Scale1To10 = 7,
  Scale1To100 = 13,
  Scale1To128inAnd1ft = 0x13,
  Scale1To16 = 8,
  Scale1To16inchAnd1ft = 0x16,
  Scale1To2 = 3,
  Scale1To20 = 9,
  Scale1To2inchAnd1ft = 0x1c,
  Scale1To30 = 10,
  Scale1To32inchAnd1ft = 0x15,
  Scale1To4 = 4,
  Scale1To40 = 11,
  Scale1To4inchAnd1ft = 0x1a,
  Scale1To5 = 5,
  Scale1To50 = 12,
  Scale1To64inchAnd1ft = 20,
  Scale1To8 = 6,
  Scale1To8inchAnd1ft = 0x18,
  Scale2To1 = 14,
  Scale3inAnd1ft = 0x20,
  Scale3To16inchAnd1ft = 0x19,
  Scale3To32inchAnd1ft = 0x17,
  Scale3To4inchAnd1ft = 0x1d,
  Scale3To8inchAnd1ft = 0x1b,
  Scale4To1 = 15,
  Scale6inAnd1ft = 0x21,
  Scale8To1 = 0x10,
  ScaleToFit = 0
}