Ordering in the right size, and making sure it fits I'm fairly tall (6'4" or 195cm), and the bike I'm looking for is fairly specialised these days (a tourer, presumably steel, possibly with a sideline as a gravel bike/off-road tourer but no need for full-on CX specs and I need heel clearance).  I ride a hybrid.  My total road bike miles add up to about the same as a decent ride on my hybrid (42 miles) and those were on a too-small bike. So I'm not familiar with road bike fit.  I do know that if I can get the saddle high enough I'll be OK for easily an hour in the saddle of anything.

Nowhere stocks a range of tourers in my size. Even finding one single tourer in stock in my size is rare (tourers don't sell fast).

Of course many places can get one in, try to fit it, fail, send it back, try again...  But how do I maximise my chances of getting it right first time?  I don't see that a rough fit on a typical road bike or even a CX bike would be very informative, as I'm after the long wheelbase, relaxed geometry of a tourer.  It feels like I could extrapolate as well from flat bars. I'd almost be glad if these assumptions were wrong, but I might take a little convincing.

The next step of course is testing it to make sure it fits -- with everything set nominally right it will feel a bit odd as I'm used to flat bars.  How can I tell whether this is bad-odd or OK-odd in a short test ride?

Part of my concern is that I'm looking at the Genesis Tour de Fer, and I'm at the top end of the largest size -- by some shops' size guides I should be OK, but not according to others (including the one I may buy it from)
The answer for a rider who's capable of doing the math themselves is use either the bikes that are available to you or help from a pro with a fit machine to establish these numbers:


Target bar height relative to saddle
Target saddle to bar horizontal distance 
Target saddle setback (relative to cranks)
Saddle height


There are somewhat more granular approaches and many more parameters on top of these, but these are the big ones to pay attention to know you're getting into the right ballpark, and will in almost all cases be enough to get you to the right size in a given model's size run.

Once you have a good idea of your target numbers, or the range of each parameter that you want to have access to in getting the fit dialed in, you can model each size of each bike you're looking at in a simple 2d CAD program. I use QCad, which is free, and getting the frame geometry of a bike into it from a manufacturer's specs only takes maybe 2-3 minutes once you know your way around. 

To emulate what a touring bike would give you using more test-rideable road bikes, you will have to fiddle with swapping stems and potentially even adding a riser for the purpose of a test ride. Some shops would happily do that for the sake of a sale, and many would not.

A bike fitter with a proper fit machine who has some sensitivity and awareness to riding styles other than racing is also an easy path to handling this. Then you're just dialing the geometries of whatever bikes and sizes you're looking at into the machine and test riding to your heart's content. There is a lot you won't learn about the bikes from doing this, but it is a very good way to learn about one's fit needs, and again it's a reliable way of knowing you're getting the right size of a given model.

One of the stumbling blocks with this on touring bikes in particular is whether the steerer comes chopped or not. The norm in the industry is for all steerers to come cut to size to minimize assembly time. On touring bikes where it's more common for people to want high positions, that causes a lot of problems. Some companies, Surly and Salsa for example, who send out a higher than normal proportion of special ordered bikes, send uncut forks with all or almost all their bikes. The important point here is that, especially being tall, you have to know how much steerer you have to work with to get your target bar height. This is true even if you were willing to put a steerer extender on your new bike, which is unfortunate when it winds up being necessary.