ATM 6" f/6

[Yvan Blom <yvan@belnet.be>]

I am grinding my first mirror, an 6" pyrex which was intended to be an
f/8 ... f/10. I was helped through my first pains (some chips broke off
the edge) by several of you ATM'ers (thanks again).
By now, I grinded with no.80 grit with long W-strokes, 2 1/2h, 1/3
W-strokes for 1h no.80, and no.120 & 180 each 1h 1/3 W-strokes.
Mirror was always on top.

Last monday, a few local ATM'ers measured my mirror's sagitta and it
turned out it would be an f/6.
Their advice was that an f/6 is not easy to figure for a beginner (and I
guess they're right...)
Nevertheless, I'd love a short tube.

What is your opinion on:
 1. If I grind with no 240 for an hour, will the f-ratio significantly be
    longer ?
 2. If I continue with f/6, what's the easiest/best way to figure it
    to a paraboloid? 
    Should I use round - square - other formed lap patterns ?
    Approx. figuring time ?
 3. Is it best to polish the mirror completely sperical before figuring?
 4. Can someone provide me with a log for a similar 6" f/6 ?

TIA,
Yvan

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Yvan Blom,  vestenstraat 65, B-9620 Zottegem - BELGIUM