Also a point to remember that Brooks touring and race saddles are actually quite narrow. You choose a leather saddle differently from a plastic saddle.
With a plastic saddle you choose a saddle that is 1-3cm wider than your sitbone width (center to center) since the padding reaches the edges and you're supposed to be sitting quite near the edges. With touring saddles I prefer even the plastic saddle to be a bit wider than normal so I use a Terry Figura which is 165mm wide with my 130mm wide sitbones.

With a leather saddle the edges are not the place where you want your sitbones to be sitting since the steel frame is there holding up the leather. The metal frame takes about 2cm from the saddle width so the Brooks B17 for example is effectively 150mm wide. But another point to remember is that with a plastic saddle it doesn't matter if a tiny portion of your side sitbone is "hanging" outside the saddle edge. Only a small portion of the sitbone is touching the saddle anyways. But with a leather saddle you DO NOT want any of your sitbone to touch the steel frame. It will feel really, really bad if that happens. So instead of measuring your sitbone width center to center as with a plastic saddle you measure outside to outside. If that outside to outside width is wider or even the same as the effective width of the leather saddle you need to go wider.
So while the Brooks B17 has 150mm of suspended leather the ACTUAL usable width number when compared to plastic saddles is nearer to 130mm. So if you can ride a 130mm wide plastic saddle you can probably fit a Brooks B17 (I'm using a B17 as an example since all other leather saddles are then narrower)

But going wider is not always an option. A Brooks b17 is already pretty wide. Going wider than that (considering there are no middle width models) can cause then issues with the saddle being too wide (Brooks b67 for example) and interacting badly with the pedaling action. I got a 205mm wide leather saddle to try out for my tourer and it was causing me electric shocks due to it pressing against my hamstring so badly. Now that saddle is on my drunk bike where it belongs.

Measuring your sitbone width beforehand is a good idea since the width issue might not present itself before the saddle has been properly broken in. When those divots appear you might realize that while you were not touching the frame when the saddle was still hard, but now that you're sitting a bit lower you are actually touching the frame. This happened to me.