It can feel awkward to send an update when the main task is still unfinished. Yet a colleague may need to know whether the work is moving and when their input will matter. Report progress in terms of evidence rather than percentage guesses.
Name the result of the work so far
“I reviewed six records and found two missing fields” is more informative than “still working on it.” It describes an observable result, even though the wider task remains open. Avoid listing every activity if those activities do not change what the reader needs to know.
Next, explain the uncertainty. Perhaps a definition needs confirming or a source has not arrived. State what you are doing about it and identify any decision that belongs to the recipient.
Give the next useful checkpoint
Choose a checkpoint connected to an outcome: a sample table ready for review, a confirmed answer, or a revised estimate. If the timing is uncertain, say what it depends on instead of offering a confident date without a basis.
Keep the update short enough to use. A practical structure is result, open issue, next step. It allows others to coordinate with unfinished work and gives you a clear record of what changed between updates, without pretending that activity alone means the task is close to completion.