Neil, I’m going to change from 32 to 36 middle ring on the front, and keep 11-34 at the back (type in the first post – should read 11-34 cassette!)
Playing with the tension and H limit screw seems to allow me to use all of the cassette in the middle ring (with the 32t middle and the 42t big ring still in place). It’s pretty close though – the cable tension needs to be just right to allow 32-11 and 32-34 without the chain touching the front mech cage. There’s also a problem if I backpedal whilst in the biggest cogs at the back, although this might be because it’s all pretty worn.
But it’s encouraging enough for me to press on with a 2×9 conversion. Not sure if I can be bothered with a bashguard. I don’t hit the big ring that often, and if I stack it going downhill then the chain will be on the big ring anyway. I might just cut the tabs off the old big ring and use them as spacers to keep the bolt length the same.