I found that by leaning the top of the ladder just below the sill, I ended up trying to crawl up the glass, pressing against it like some middle aged "Spiderman"! [Own up, the person that shouted "old git"!!]
I recently had a problem with bay windows that have pitched roofs; the ladder rests on the gutter before it reaches the wall on the first floor. I went out and bought a "Ladder Max" to help me . This is a device that stands the top of the ladder away from where it would normally rest.
It works so well with bays that I left it on the top of the ladder, [about the third rung down, actually], and used it to space me away from first floor windows. Very good for those occasional tall windows with nasty convex gloss painted sills, I'm 5 foot 7".
The "Ladder Max" is plastic and a bit heavy, but I liked it in principle. I still use it for bays, but at almost all other times, my ladder wears a "Wickes" aluminium stand-off. It's a bit lighter for general use, and you're not so far away from the wall, as you are with the other device.
Both devices stabilise the ladder by a hell of a lot, and as far as I am concerned, these are now a [permanent] part of my risk assessments.
Cost? "Ladder Max" £28 and the "Wickes" device, which is a "Youngmans" product, £22.99.
I wonder if this helps?
Baldeagle. [Stone/Stafford]