i agree with peter that you need plenty of drones flying, and i would put the cut down split on it's own stand.
i am a little bit south of you, and it is too early here. i'm not sure it's even warm enough for them to draw new wax yet.
mike bush says the best time to do a cut down split is just before the main flow, which coincides with the peak swarm season.
walt wright describes looking for signs of broodnest expansion vs. broodnest reduction as a way to assess swarm intent.
if you see an band of empty polished cells (that have been prepared for the queen to lay eggs in) at the edge of the broodnest, that would indicate they are expanding the nest and not preparing to swarm.
if you see the cells at the edge of the broodnest full of nectar, it means they have stopped expanding and are reducing the broodnest. they will also start filling the cells in the middle of the nest as the new bees emerge. swarm cells and swarm issue are soon to follow.
the idea is to do your cut down split before it gets to that point.
i learned last year that the main flow here coincides with the tulip poplar blooming. it's a good idea to journal the timing of the blooms in your area and compare that with when you see swarming. it will give you another measure as to when to do your spring splits.
beekeeping since june 2010, +/- 20 hives, tf