tentative points

hello everybody,

i'm just a total newbie in macro programming so this is probably a
stupid question...
i just played around a little bit with some examples and the macro
recorder...
i do not understand how to pass a tentative point to a macro. e.g. the
following (recorded with the macro recorder and modified):

Sub main
    Dim startPoint As MbePoint
    Dim point As MbePoint, point2 As MbePoint

    MbeSendCommand "PLACE LINE CONSTRAINED "

    startPoint.x = 72.065966#
    startPoint.y = -115.007869#
    startPoint.z = -0.000035#

    point.x = startPoint.x
    point.y = startPoint.y
    point.z = startPoint.z
    MbeSendDataPoint point, 1%

    point.x = startPoint.x + x-value 'calculated by some subroutine
    point.y = startPoint.y + y-value 'calculated by some subroutine
    point.z = startPoint.z
    MbeSendDataPoint point, 1%

    MbeSendReset

    MbeSendCommand "PLACE LINE CONSTRAINED "

    point.x = startPoint.x + x-value
    point.y = startPoint.y + y-value
    point.z = startPoint.z
    MbeSendTentPoint point, 1%
'  recorded- clicked snap icon- clicked snap center
    point.x = startPoint.x + 4.392452#  'how can i get here the result
of the snap ?????
    point.y = startPoint.y + 1.783401#
    point.z = startPoint.z
    MbeSendDataPoint point, 1%

    point.x = startPoint.x + 7.904211#
    point.y = startPoint.y - 2.047609#
    point.z = startPoint.z
    MbeSendDataPoint point, 1%

    MbeSendReset
End Sub

this should be drawing a 2nd line starting in the center of the first
line
but how to hand over the center point? 
who can help me?