Try this
- Name the range of uncertainty and any decision that depends on the finish time.
- Choose a plan that allows the waiting person to proceed independently. Send an update when a more reliable time becomes available rather than promising certainty early.
A worked example
“I may finish between six and eight. Please start without me if needed.”
An invented example to adapt, not a real exchange or a script you have to follow.
Before you leave it here
Update when useful information changes, not merely to repeat that you still do not know.