If you created the partitioning from AmigaOS then you see the
"disk" geometry in HdToolbox and can calculate the start by
multiplying LowCyl * SecPerCyl = LowCyl * nHeads * nSectors.
The alternative is to use
http://ftp.netbsd.org/pub/NetBSD/misc/mlelstv/rdbedit.tgz.
with a slightly cryptic user interface. It lets you
- create an AmigaOS partition table (RDB) starting on blkno 2.
- dump the table as a disktab that you can use with disklabel -w
to write an additional NetBSD disklabel to blkno 1.
The result is a medium or image that is understood by AmigaOS, NetBSD/amiga
and other NetBSD archs. If you don't want to go through the experience
of miniroot.fs, you can also unpack the sets and complete a bootable image
in a few steps on your non-Amiga NetBSD box.