OK...on my Raleigh mixte there is currently a Sturmey Archer stem 22.2 x 25.6. I understand those measurements. 'Assuming' you measure height from the pointy end to the top of the column, mine measures at 145mm with a 90mm extension (approx - I don't have calipers). I would like the stem to be taller. I've looked at the Nitto technomics - they all have 225mm columns with the extensions being different lengths. Am I thinking correctly that the extension lengthens/shortens the reach? How will I ever figure out which one I want until the entire bike is built up with the seat adjusted correctly, etc.

Any help getting my brain around this would be greatly appreciated.

Your calculating sounds right to me. And it is hard to figure it out unless you've worked out the other details, particularly the position of your seat and your bars. Best you can do is deal with one component at a time; if you know you need a taller stem but the reach currently feels okay, go for that.

Neal

Can you lower the seatpost at all? If you can do this, you can set up the saddle to the lowest possible position, then raise your existing stem to a position which feels comfortable relative to your current (lower) placement of the saddle. Then you can see how the handlebar you already have feels... perhaps either too close or too far... and then order accordingly.

Later, when you raise both the saddle and stem they will retain basically the same angles. So, the distances will remain about the same. Of course, you will ALSO need to use the same handlebar which you intend to use later, because a different style bar can very dramatically alter the overall reach - just as much as the stem extension.