The distance between the ischial tuberosities (sit bones) of males in the ninetieth percentile varies by not much more than 4-5mm - less than 1/4". And it's quite likely they're not even symmetrical.

However, the tuberosities are not really points but two ridges arranged in a vee-shape which means that the distance between the effective points of contact on a bike seat vary according to the rotation of the hips. This is very much dependant on the rider's flexibility, age and general body type and can vary by much more than 4 or 5mm.

Using those 2D flat pad devices cannot replicate the degree of hip-rotation you might - or might not - achieve on a 3D bicycle saddle and results are meaningless in the real world.

I've just had the opportunity to ask a couple of designers and manufacturers of high end bike seats why they don't provide some sort of measuring device to guide purchasers to the right saddle for them.

The explanation in all cases was pretty much as I've outlined above.

Of course, generally it's true that those riders with exceptional hip-rotation (think of L@nce Armstrong) can ride an very narrow seat and mere mortals like us may need a wider one. But several years of association with expert bike fitters has shown me that there are plenty of exceptions to every rule when it comes to saddle comfort. I've see beer-bellied 60-year-olds happy on super-slim rails and near pros happy on a touring saddle.

I am happy for those riders who find bliss on a $15 plastic saddle as many do, but, if seat comfort is an issue for you then guidance and fitting by an expert who can assess your body type and flexibility is probably the only way to go. And be prepared for sticker shock.

Now, if you're going to ask what works for me, I'd say the Fizik Aliante and the SMP Dynamic are dependable and extremely comfortable. But I have a Trek OEM on my mountain bike which probably cost $15 and that works pretty well too. I've tried plenty of others which are torture.