I see that you can add margin in pixels to the top or bottom, and % in the left/right, but is there a way to add a pixel value to the left/right margin?

I think the answer to that would be,,,,,, only after exporting and then messing about with css

Is there a reason that it's not available beyond it just hasn't been added yet? or requested yet?

I'll submit a feature request but won't if there is a specific reason it's not in there already.

Thanks for the reply!

The reason is most likely that, if you have a px value, it won't change to adapt a smaller (or larger) screen. You would have to adjust it at a lot of breakpoints all the way up/down. With a margin given in %, the margin will adapt to screen size. Clearly, you might need some breakpoints there too , but by far less than if you use a px value.

From the latest tweet from CC it looks like we'll be getting new value settings for many areas.
http://i.imgur.com/pSUyEcy.jpg
I hope that includes padding/margins as well. I understand what you are saying regarding the responsiveness but I would rather have that control be my decision as needed. Some designs will require margins not to change, if I need more break points occasionally; so be it.