New Bike based on stack and reach

As you say that you're happy with your current fit, I'm going to assume that a bike fitting isn't something of interest to you.  You might want to consider it though, to validate your position before you go and buy something expensive.

I quite like using Stack and Reach, although there are other factors.  One of the gotchas for me is that you don't strictly need stack and reach, you need 'effective' stack and reach, by which I mean the distances to the bars, since none of use sits there holding onto the headset.  Here's how I work it out:

- Preferably, use a diagramming tool like Visio or a cloud equivalent like LucidChart (it's free and just runs in the browser), get familiar with the ruler and the ability to place marks on exact co-ordinates

- Mark a point on the diagram for the BB - just use a coloured dot, and if you can, place it at xy 0,0, this makes it more intuitive to then work with the stack and reach figures

- Refer to the frameset geometry of your current bike to mark the stack and reach to the headset of your current bike

- Combine head tube angle with amount of headset spacers, length and angle of stem, and reach and drop of bars.

- Armed with this info, mark on the diagram three different 'effective' stack and reach points - one for the bar tops, one for the shifter hoods, and one for the drops (you'll need to make an assumption regarding the rotational angle of the bars and the position of the shifters)

- Change colour, and repeat for the geometry of the new bike you're lusting after - reuse the existing BB mark, and you will see how the new bike will differ from your current one

Yes, I know this seems really fussy, but if you're dropping a couple of grand (or more) on a new bike, you want to know that you'll be comfortable in each position.

Lastly, I bought a Genesis CdF online, based on the geometry diagrams.  The thing I hadn't factored in was that I pedal with my heels turned inward slightly, which is fine on rim brake bikes, but the CdF has disc brakes for which the chainstays are flared.  They catch my heels and I had to sell it on.  Just an example of the kind of stuff I would have picked up straight away on a test ride.

I did this for both recent bike purchases and it certainly helps, because it should mean you can get to an acceptable range of positions using different stem lengths and possibly seat post laybacks. You need to take a few more measurements though, there are quite a few geo comparison apps available online. For me due to knee issues seat tube angle (and therefore saddle setback) is important.
If you're super-comfy, measure everything - shifter position, saddle height relative to bars, etc. Whilst positions evolve over time having a set of measurements from a known-comfy bike is a great baseline, you can spend ages chasing the right position on a new bike.

I can attest having done this for all my bikes that setting up using stack and reach really works.

You'll need a spreadsheet, enter the geo for your current bike in the column B an each of the bikes you're interested in in successive columns, and compare (with a simple (original bike - bike x) sum.

If you include a figure for stem length, you can work out what length of stem you'll need for bike X too.

Of course, they don't *feel* the same, but you'll be sure that you'll fit the bikes like a glove.

This only applies for one type of bike (eg road bikes), if you want a tourer or something completely different from your current bike, then road test them.

My suggestion would be not to base your next bike decision on a couple of dimensions on your current bike.

I have three bikes; CX, endurance road and a road (racey) bike, none of the dimensions are the same (they are all marked as 54cm) and they all fit me.

You're probably better off looking at a bike that is marked as the same frame size (medium/54cm whatever), depending on the type of bike it is, you may need to change the stem, saddle position and bar width.

Road.cc had canyon at their demo days last year (more planned for this year), if you can this is a great place to try the sizing out. I think Canyon can be a little odd with their sizing, my mate has an XS race spec frame and it's got a taller head tube than my larger size TCR bike. 

Stack and reach are a very rough guide to fit. There are a lot of other geometric variables, that contribute to the fit of the bike. I tend to look at the classification of bike, then the frame size first. Then I’d look at how each bike types stack and reach compare for each type. Forex, a full on TT bike isn’t a ‘sportive’ bike, and a Sportive bike ain’t a hardcore ‘aero’ bike. So don’t try to compare apples with oranges to start with, and it becomes a bit easier.