The % is determined from 120% of the federal mid term rate ( no more than ).
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from Fidelity that includes the current rate. I think you can also look back to the previous rate if that is to your advantage.
As far as dipping in - you can divy up your ira into smaller ira's but you don't want to mess with the IRA at all that the 72t is taken from - this can be a very taxing event. If you split your IRA into several you can start a 72t on one of the untouched funds if needed.