We don't use Phases so I have a slightly modified way to look at it. This might not help when using RAW.
HR: If you get a Partial Success in SCSM, and you choose to keep casting, then the spell just takes an additional 50%Act.
If you think that %Act is actual time, some GM's run this way, then an instant in the second round might not help at all. If you want a Blade Turn for this round, then it might not help.
If you play that %Act is everything you can do in a round, not time dependent, then you could still get that Blade Turn up if you still have 50%Act. Haste, for example, grants additional %Act so the spell just eats up an additional 50%Act. Now you can get it off this round.
The caster can choose not to keep casting, using their %Act for something else. I still count the Partial as casting a spell for spell/round limits.