VB中cad二次开发的问题，望高手回答

在实际操作中，A3的可打印大小约为407.6×283
可是我编程得到的大小却大些，约为413.8*290，这样导致有些东西没打印出来。
为什么？怎么解决？
Dim MarginLowerLeft As Variant, MarginUpperRight As Variant
Dim PaperHeight As Double, PaperWidth As Double
Dim PlotHeight As Double, PlotWidth As Double
thisdrawing.ActiveLayout.CanonicalMediaName = "A3"
thisdrawing.ActiveLayout.RefreshPlotDeviceInfo
thisdrawing.ActiveLayout.GetPaperMargins MarginLowerLeft, MarginUpperRight
thisdrawing.ActiveLayout.GetPaperSize PaperWidth, PaperHeight
PlotWidth = PaperWidth - (MarginUpperRight(0) + MarginLowerLeft(0))
PlotHeight = PaperHeight - (MarginUpperRight(1) + MarginLowerLeft(1))