It’s great how instead of sidebearing’s value, you can put in a parent value. And changes apply to the child. But what a child sidebearing is only slightly different? For example: left sides of *h and *l, when *I has small hook at the bottom.
What if you could put in “h±XX” value?
Then you put in “=h+15” 
Don’t forget the equals sign at the beginning.
Have a look at http://vimeo.com/glyphsapp/spacing
Oh wow. Didn’t know about placeholders and pipe trick.
what I actually still wonder about all the time: could it take different values for each master, as it is already implemented (e.g. in the round corners filter). Actually one does not want only +20 in a very light weight, if one has +20 for the black.
The different values for different masters are not jet implemented.
Takes the value of b and multiplies it by 1.2, i.e. increases it by 20%. You can add, subtract, multiply and divide.
By multipling parent’s siedebearings you can also quickly space small caps, denominators etc. it is really usefull.