What size spacer do I need?

I have a 2012 e92 M3 with Competition Package and I am planning to run Apex 19x10 wheels all the way around but only at the track. The Apex wheels have 25mm offset which is the same as my ZCP rear wheels but my front stock wheels are 31mm offset.

What size spacer do I need to make the Apex wheels work on my car for track application?

Thanks.