The best way I know of figuring out a way to do stuff in Word is by
recording a Macro and then viewing the generated code. It's VB, but it's
pretty easy to translate it into C#.
In Word 2007, if I record a macro that changes the Start at you're talking
about I'm getting the following macro -
' Macro1 Macro
.NumberStyle = wdPageNumberStyleArabic
.HeadingLevelForChapter = 0
.IncludeChapterNumber = False
.ChapterPageSeparator = wdSeparatorHyphen
.RestartNumberingAtSection = True
.StartingNumber = 3
You're only interested in changing the StartingNumber property and the
translation into C# is straightforward.