You really don't have to make an appointment, you should be able walk in and get measured it only takes seconds. I didn't write the below statement but it actually explains the seat fitting very well.

"The idea of the sit bone measurement is to match you up with a saddle width that allows for your sit bones to be adequately supported by the width of the saddle. If you use too narrow of a saddle, your sit bones will be placed towards the outer edges and may not leave you adequately supported. This is especially true as you slide forward along the saddle during a ride. If you get too big of a saddle, your sit bones can be placed too far inwards towards the cutouts, which may not leave you adequately supported. And sitting too close along the cutout portion actually can cause more comfort problems for the typical rider than anything else with a cutout saddle.

What has worked best for me is to get the width that allows me to center my sit bones on padded ovals. That way I can move forward or backwards along the length of the saddle as needed and have the best range of sit positions, while still being adequately supported.

Something like 70% of riders will fit nicely on the middle sized (143mm) saddle, according to the Specialized rep I bumped into at my LBS. Your LBS should have a sit bone measurement pad. You sit on it and compare the impressions from your sit bones to the dimensions of the three saddles and go from there."

I ride the Avatar and have no complaints at all. The key though is to get the proper fit. Without it, they won't work out as well.

Hope this helps.