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blanking differences/sizing

I'm having some sizing questions about S16, that is actually an issue 
with all formats. Working in 625/PAL the nominal hor. blanking is 
11.95usec. The 4x3 and by extension 16x9 aspect ratios (as far as I can 
tell) are calculated using this blanking. 625 blanking for CCIR601/RP125 
is 10.6usec. In order to frame the picture with correct aspect and a 
maximum amount of scanned film area (using BKSTS/channel 4 film) one 
would reference to the 11.95 blanking. This obviously gives frame edges 
in the picture when viewed with CCIR601/RP125 blanking. If you simply 
zoom in do you also re-center, as the added picture area is not equal on 
the front and back porches? Do you keep the same center and zoom in 
until it clears the back porch. Are there repeatable standards, or just 
"we do it this way" or "this client wants it that way? Any and all 
recomendations would be appreciated. 
	One other issue: This test film shows a 4x3 aspect with the 
same upper and lower limit as the 16x9. As the film is 15x9, you 
obviously loose available image in the top and bottom of the screen. I 
could not understand the reason for this until I considered the need to 
transfer a 16x9 Lbox, or widescreen 525/625, do a DVE conversion to 4x3, 
and do a matching transfer direct to 4x3. Any comments about this loss 
of image area?