I have my saddle height and angle set. What im trying to figure out is how to measure a saddle to fit my boney butt. How do I figure the right width of a saddle to fit my butt correctly?

Go to a Specialized dealer and use their body geometry sit bone measurer thingy.

Or a piece of tinfoil on a carpeted stair works pretty well.

If I do the tin foil trick, what do I measure?

Place the tinfoil on one of the steps, sit down on it, lift your feet and rock back and forth a little. Try and find the center of each divot made by your sitbones, and measure the length between the two. Use the narrowest seat you can, while still being wider than that measurement. Make sure not to wear jeans, or something thick.

So if you measure 125mm, use a 133, if you measure a 136, use a 143, etc.

You could also find a dealer that does the fizik test saddle program. At first I thought the spine concept thing was extremely stupid but to my surprise it's actually pretty acurate, for me at least. Found out i'm a chameleon and haven't thought about changing my Antares VS since.

Riding posture also matters. Generally, if you sit more upright as you would on a touring bike you will want something wider vs. a narrower saddle with a forward racey posture.

This is true. I prefer a 133 on my track bike, my usual road 143 just wasn't feeling quite right. Same saddles btw, just different sizes. And the 143 is a goddam sofa on the road.

I guess i would be in more of a mountain bike posture when riding. I tried the foil trick and if im measuring right im seeing around 140mm.