Problem with multi-SF predecessors

Suppose we have 3 tasks, A, B and C. task C should start before both A and B. If we give C a A-SF and B-SF predecessors, C will begin before later one, but I need it to start before earlier one:

Result:

A =======
B .............========
C .............^ (A-SF & B-SF)

What I want:

A =======
B .............========
C ^ (?? & ??)

(I’ve shown C with ^ because I was not sure how spaces would be shown in others’ screen. Imagine C is a milestone, but tasks would act the same).

I don’t want to remove later task’s relation, because I do not know exactly which one would start earlier, and I want a dynamic plan.

The same goes for SS predecessors. I can not make C a ALAP one, because in that case, C would push all successors in their slacks (floats).

I thought about scheduling from finish date, but it is not compatible with plan’s specifications.

Is there ***ANY*** solution for that? No matter how complicated it is.
I think there’s a serious need for SFX and SSX relations (and there would be no problem with having FSX and FFX relations too). By the way, the usual ALAP constraint will use all the Total Slack of the task; it would be nice to have a ALAPX which use only the Free Slack.

I usually use MSP, but I’m used to use Primavera besides.


Thanks