Theoretically, a 50% split in volume of wort collected between initial mash (1st runnings) and sparge (2nd runnings) helps to optimize extraction of the sugars create by the mashing.
In terms of BS, it will do whatever you want. Reconfigure the mash profile to match how YOU want to mash. The existing profiles are included to give the users initial profiles on which to build or modify for their own means. They are not recommendations or a depiction of how you should brew. This is one of the major attractions of BeerSmith, the ability to personalize the profiles to match your method of brewing!