WRT the "set" length of housing loops. Actually there is a standard length, at least for Shimano rear ders, if all the aspects are within Shimano's specs. The big S has published recommended specs for pretty much any/all elements that influence the rear der housing loop length. Like how far from the hanger mounting hole is the chain stay stop and the angle WRT the chainstay that the hanger's "B" screw stop is at. So if the bike has followed all the little details as Shimano wants one could order the "set" length off a supple sheet and know that as your thousands of bikes were being assembled by the 12 year old girls in Asia (please take kindly to my poor attempt of a joke) will have quite acceptable housing loop lengths.

But in the real world of shop or home installing this detail isn't to be known, followed or assumed that the OEM assembly got it right. Hence I cut the housing length an inch or so too long , test fit and then trim down to what I've learned works well. Smooth and even curves with the ends of the housing entering the stops/der adjusters without any kinking or doglegging. And these days I find a touch longer the ideal is a better side of the fence to fall on then a tad too short. Andy.