For those who develop their routings using a separate tool (such as FPP) there is EuroFPL which is what I use. They offer the Eurocontrol "route suggest" facility which is usually OK, but the FPP routes tend to be better optimised.
EuroFPL also remembers successful routings made by other people, which means that between most airfield pairs there is a choice of half a dozen routings found by others, each declaring the percentage distance more than GC.
I must say that I tried FPP and didn't find that it worked (it was very buggy, that might have improved since) whereas EuroFPL usually gets me around 105% - 115% GC with no difficulty.