Honestly it's totally dependent on your playstyle and what you're comfortable with; since your role isn't a mana battery, I wouldn't worry about stacking more spirit you feel you need.
Hard to put an exact number on it, depends on so many factors. If you use Totemic Recall / Telluric Currents Glyph, if you have the meta, how many other Mana Tides & Priest Hymns there are, your role & playstyle, etc.
You could still feel fine with your 13.5k. I'm 10/13h 25man right now and run with 11k spirit; works for me but so could a wide range of numbers. Have fun with the 10->25 transition