The best way to work out this question is by trying. If the string is too low the player does not feel enough resistance from the fingerboard and will quickly learn to step too lightly on an F with the result the string keeps vibrating under the finger. If the string is too high the player feels enough resistance, presses harder and also more of the nail region gets involved. Some players learn themselves to play "on the nail" all the time. Helps with intonation because it stops the string dead but same time produces a nasty quality in the tone. From what I read here Mr. Michael Darnton seems to have the right idea. Basically, making the violin too comfortable in the first half position might not be the best thing. Same way, having strings too high bridge side will eventually lead to some form of injury as our tendons become calcified with age.