Gilbert,
In a case like this you need to do the best that you can and estimate. I had to do exactly the same when I filled in my application. My company didn't have an accounting system at all, so it was even more difficult.
My recommendation is to estimate and then give your estimation to your supervisor(s) for review. Ask them if they agree with your estimation. That way you can be certain that you estimated right and that they will approve your hours should there be an audit.