manual steering box oil level

[col320ce]

I've just put the steering box back in and the manual says fill to 32mm below the filler (which I assume is the raised area on top of the box).  The bit that I think is the filler is stuck and I don't want to force it however the bolt that covers the centering pin is accessible.  Is the box pretty much filled to the brim if I use this access site?
Thanks again everyone ...