what is an offset (in sectors) between beginning of partition used to make
raid and raid data stored on it

as bootblock in 1.5.1 doesn't support RAID-1 boot i would like to make
"shadow" partition just for them like that on both disks:

a: <raidsize> <e_begin+offset> 4.2BSD ....
..
..
..
e: <raidsize> <e_begin> RAID .....
f: 200       somewhere 4.2BSD

and install bootblocks (nothing else) on f, which then will boot from a
not knowing that it's on RAID-1