Activity Relations

Activity Relations as we all know are listed below

1. FS-Finish-to-Start
2. SS-Start-to-Start
3. FF-Finish-to-Finish
4. SF-Start-to-Finish--> which im not using in my schedule.

Why dont we develop relations like

Act A

---> SS Lag= after 50% of Act A

Act B

Because the start of most activities will not depend on the start of predecessor activity but depend if that predecessor activity completed 50% already.

Is that logical?

Thanx and more power,
Francis