I just put the numbers in on the common gear setups we are talking about here. I’m assuming a few minor details like everyone has 700-32 wheels and that those using a 10 speed cassette are using a 11-36 that goes 11 13 15 17 19 21 24 28 32 36 and those like myself with 9 speeds are using a 12-36 that goes 12 14 16 18 21 24 28 32 36.

We have those like pacificcyclist, nun with doubles of some type with gearing such as 42, 24

Then we have cyccommute , cplager etc. with true triples 46,36,24 or 48,38,22 respectively. And others with triples of the 44,32,22 or 42,32,22 variety.

Then myself with myself with my half step + granny that’s somewhat a clone of the two methods 45,42, 24

Everyone seems to agree a 22 or 24 tooth granny is what is needed and they will give you a usable range of low gears 17 to 35 GI about without cross chaining so there is no question that small ring is needed no matter what the other ring s are.

Now the 42 tooth ring and its range assuming it can be used across the whole cassette for the 10 speed its 31 to 103 GI and for the 9 speed it’s 31 to 94 GI because of the missing 11t cog.

For the 36 tooth center ring its 27 to 88 for the 10 sp and 27 to 81 for the 9 sp.

For the 32 tooth center ring its 24 to 78 on the 10 sp and 24 to 72 for the 9 sp.

I have tried the mountain cassettes and for me it boils down to cadence and mine was too slow to support a center ring as small as 32t. It covered about 2/3 of what I wanted that one ring to do, and in my case I wanted my center ring to cover the widest amount of my normal riding kind of a RMS value of what I do. If I wanted my gearing to be split between two rings something like a 22, 26, 46 would have worked for touring. Half my average gears would be on the center ring half on the big ring. But that put my sweet spot the honey hole of GI right on the shift line. What it would have done is tighten my spacing between gears. The idea for me at that point and discussing this with nun and some of the guys here was to put my sweet spot on the center / center position thus minimizing my front shifts. Albeit was going to be a tougher shift to the granny, it would widen my spacing, and I didn’t really then have much use for the big ring. In experimenting loaded and unloaded I found the 42t was that ring for me if I was just looking for a loaded touring ring I may have selected 40 or 38. If I had went strictly touring the 38,22 that cplager has would have been very nice. But the question still is what to do with the big ring. He has 48 38 22, very similar to what I was riding around with 52 42 24 and his 48 gives him just 2 more gears higher and 4 more one and a half step gears to fill in some gaps similar to what I had. His front shift is a 10 tooth jump so it wouldn’t be too bad to shift to get those two gears but I wouldn’t see myself doing it to get a one and a half step gear unless I really was going to be in that gear a long time. If he had the 11-36 crank his top GI would be 93 GI on the center ring and he would still get 2 higher gears but most likely one would be too high. In pacificcyclist case with the 10 speed cassette he’s seeing a range of 31 to 103 with that single ring I can see why he’s happy with just that double crank. The difference of adding that extra 11t cog in there made the difference filling out the full range for him as long as (one) he doesn’t find the spacing between gears to great and (two) 31 gear inch is low enough to cover his majority of riding. In doing that I know he has a double technically speaking but to me it’s a wide range single with a granny that’s unrelated or a bail out gear. The thing I liked about a wide spaced cassette is that if I only use 6 of the cogs with my granny ring I can have a wide range of granny gears so I don’t have to shift out. When I’m nearing my largest cog on the center ring I know the way to shift down is to move over in the back to someplace close to the center. Then drop on the granny and I’m in a close gear as a starting point. If I had a 10 cog cassette starting at 11 thru 36 I might have stopped at a double. With the 9 cog cassette I was one cog short as I started with the 11-32.

Now that I have the half step I couldn’t be happier with the triple. In the old days on a 2x5 you had to move from a 42 to a 52 ring to make a half step shift. Now I only have to move from a 42 to a 45 to get half. But for me it’s also about chain line. When I get on the outside ring I have 45,12 45, 14 and 45, 16 101, 87, 76 GI that are silky smooth. On the double of 42,24 double guys they get 42,11 42,13 and 42,15 for 103, 87, 75 GI I don’t know if that chain line is as nice. I know when I had the 11-32 on the center ring to the 11 wasn’t anything as smooth as my outer ring now to the 12. That and being able to split any two gear inches in half with single shifts is well worth the extra ring.

When the OP asked about a double I wonder what rings were going to be suggested for him as a double. We all know a 42,24 works ok but I don’t see a bike shop recommending that as a new bike drive.

As strange as it sounds with my setup I don’t think I would want to go to the 10 cog cassette I think 9 and the spacing it came with worked out perfect for the double + granny.