Don't focus on the ambiguity, focus on the concreteness
[alf].
Lock the parts down you can, getting a larger and larger base of concreteness.
That foundation will allow you to reach further into ambiguity.
You need something concrete to lever off of to stay strong in ambiguity.
A mistake a lot of people make is to focus on the ambiguity instead of what can be easily nailed down.
Ambiguity has compounding cost, so nailing down more pieces makes remaining decisions orders of magnitude cheaper.
Nail down the most obviously true, inescapable parts first, and work your way down to ever-more debatable points.
As you work your way down and get more real world feedback, it will get easier and easier per unit ambiguity.
Don't postpone decisions you don't need to.