I that you were supposed to start in the middle of the button.
possibly they are giving a little extra seat room because of a deep seat??
several of the dressage saddles tell people to size up a seat. or a half size. that may be that saddle makers way of compensating for that.

 

I've always measured from the front of the button and included the button within the measurement.

 

Ok, I'll go with including the button.

I've been seeing way more 17" saddles on the used market than 16.5" so that's probably the size where I'd need to start looking anyway

 

I think your reasoning is good. I always seemed to measure from the middle of the button, but I think you do include the button. Funny, who'd think it made much of a difference.

 

This is what Ebay says about it:


I have found a lot of sellers that have been measuring their English saddle like a western saddle. This is incorrect. If you measure your English saddle from the center of the gullet straight back to the cantle, the size comes out way too small. To measure an English saddle, go from the brad or nail head at the side of the pommel [front of the saddle] to the center of the cantle [back of the seat]. This measurement will be at an angle. This will give you a correct seat size.
When purchasing an English saddle, most people think the seat size is all about their size, but the most important factor is not your fanny size, though that plays some part in fit, it is mostly the length of your upper thigh. If you purchase a saddle that is too big for you, you will have a very hard time ever maintaining a correct seat position, you will constantly have to fight to avoid riding chair seat, and your posting will not be the small, smooth movement it should be, but rather a huge effort at popping up and down in the saddle. When you sit correctly in any saddle, [with the exception of a saddle seat saddle], you must have a line from your shoulder to your hip to your heel.

 

Hey OP I think the size of saddle you need goes by your hip to knee measurement. You sit down in a chair that leaves your thigh parallel with the ground and measure from the back of your fanny to the end of your knee and that measurement correlates with different saddle sizes. Also keep in mind that you have to fit your HORSE (not only your bottom) and not only as in narrow, med or wide but the LENGTH of the saddle as well. Many short backed horses have to have special saddles to conform to their short backs. Just a few tips to hopefully save you the headache of buying 10 - 20 saddles before you find the right one (like yours truly LOL)

 

Thanks for the tips WildAcreFarms I'll definitely be making sure the saddle fits both me and my horse!