You can handle the grid.Model.QueryColWidth event. If the e.Index points to your floating column, you can set e.Size to be the size that you want the column to be. If floatCol is your float colindex, you can try code like:
if(e.Index == floatCol)
{
e.Size = grid.ClientSize.Width - grid.Model.ColsWidths.GetTotal(0, floatCol - 1) - grid.Model.ColWidths.GetTotal(floatCol + 1, grid.Model.ColCount);
e.Handled = true;
}
Here is a link to a KB that gives a helper class that offers a more general solution but is a little more involved.
http://www.syncfusion.com/support/kb/grid/Default.aspx?ToDo=view&questId=304