Best not to have top or bottom fixings if you can help it. Foam and sealant where possible for those.
For side fixings at least 150mm from the corners and then approx 600mm spacings.
I would take the cill off and check it is siliconed to the frame. Back edge of the cill and both of the sides where the frame ends. You would be annoed if you fitted it and it leaks first time you get heavy rain!
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