Use the "tell" version, not the "knew" version.
The phrase, "as far as I know" is an idiomatic expression meaning:
- I may not have the whole story, but from what information I have, I can say...
Moreover, the phrase wouldn't be used with the word "you" tacked onto the end. You could say:
As far as I knew, you were a good manager.
This means that you thought the person was a good manager, but there may have been other people who would disagree with you. Or, we could say:
While I knew you, you were a good manager.
But you would not say:
As far as I knew
you, you were a good manager.
The phrase "as far as I can tell" means roughly the same thing, you omitted the "you," so but the version with "tell" works a lot better with the part that follows:
As far as I can tell from working with you for a short time, you are a good manager
That all said, expressions like "as far as I know" or "from the short time we worked together" are qualifiers that cast doubt on the person's management abilities. If you want to laud the person, you should just say:
You are a good manager.
The other parts shouldn't be used unless you want the reader to know that your experience is limited and therefore may not be accurate.