Position a label under a datagrid column

Question

Hello community,

 

please help me. I'm sort of despair... I try to position a label under a datagridcolumn in WPF. And I can't find a good point to get the true left start of that column. The positions can't be fixed because it shall be possible to reorder and resize the columns. OK. I'm also a little new to WPF...

Can someone help me with a possible solution?

Thx.