check out the MidSpey specs on RIO's website
this info should get you started as it indicates where the cuts are made on the factory lines:
Vincent's specs show the cuts made at @ 15-1/2ft and 27-1/2ft. Generally the second cut is the attachment point for the "serious" sink tips, the RIO "Big Boys", while the 15ft point is for shorter tips (like 15 footers) of various sink rates.
I guess two important questions are 'What sort of tips are you planning to run'? and "How experienced a caster are you'? You mention 15ft as tip length, but a 15ft type 3 and a 15ft type 6 are potentially two different animals as far as casting them is concerned. And if you plan to use 15ft lead core (a friend of mine calls it "dead core") then such tips will make casting even more interesting.
The 22ft cut point you've been advised on is well into the 2nd level section of the line and should allow you to turn over pretty much any 15ft tip you'd care to throw with good technique. If I were a fairly new Spey caster I might seriously consider looping my line further into the belly (at the 22ft point). But an experienced caster would probably feel very comfortable following RIO's diagram and putting the loop at 15-1/2ft.
I'll also contact Simon Gawesworth at RIO and see if he can respond to this for you as well (BTW--take his advice over mine and pretty much anyone else's when it comes to RIO's lines!)