My tactic when working split DX is to tuck myself right down at the bottom and just sit there and call.
If I call in the middle of the split QSX there will be other stations calling on either side as well as on top of me. The top end of a split QSX is usually very variable with no defined upper limit. But down at the bottom there will be less stations calling on the LF side of me and sooner or later the DX will be listening at the bottom end.