A calculator cannot remove uncertainty, but it can show you where uncertainty matters.
Use a rounded, believable figure and run the scenario. If changing that estimate a little does not change the decision, it is already good enough. If a small change reverses the result, that number deserves more work.
Precision should earn its place
Spend effort on the figures that drive the choice. Do not delay the whole exercise to perfect a minor cost that changes nothing important.
What to try next: Test the uncertain number 10 percent lower and higher. Notice whether the conclusion moves.