you have to use arccos, the inverse consine function. it gives you radians (0…2Pi rad ^= 0…360 degree), which is the mathematical measure for angles. divide the radians by 2Pi to normalize it (then you have the vvvv angle measure 0…1), then multiply by 360 to get degree…
not long ago there was a thread about angles here.
gregsn posted some modules dealing with angle conversion.
unfortunately he attached them to the forum instead of using the wiki.
well, vvvvdevs seem to be pretty busy.
note that if you deeply understood Point2Vector, Vector2Point and the power of the phase input of the CircularSpread, you can avoid almost always dealing with sines, cosines, arctans, and the like.
similarly Map is made for never ever thinking again about rules of proportions (rule of three) and doing things like converting from rad to grad to angles to whatever always cleanly with just one node. So i not really recommend focussing on gregsns little modules. basically you can always invent the solution on the spot with very little clicks after you know some basics.
and most of the time you even don’t need the map nodes, cause all rotations are measured in fractions of full cycles. however the expr e.g. does ist otherwise. and entering 2*pi into the map-node can be pain.
so, that’s the most important thing: you always should be aware of the value range of your data. then you should be able to do the convertion on your own.