Considering that you have been at home with your children for the past 7 years and homeschooling your eldest, he's going to have a hard time proving that it is in the best interests of the children for him to be awarded custody.

The courts don't look favorably upon uprooting children and disrupting their routines. Unless he has an immediately necessary and highly compelling reason to do this - and no, "My mother will take care of them!" is neither immediately necessary nor compelling - he's going to have a rough go of it.

Michigan Custody Guidelines are here.

With respect to child support formulas, it's going to depend on when you file for Separate Maintenance. If you file before September 30th of this year, the 2004 formulas are to be used. After September 30th, the 2008 formulas apply.

The formulas, manuals and calculation software are available here.