"correct" beats will summon the jazz police :). The more sophisticated your playing becomes, the more liberty you will have with how the chord tones or alterations fall within the measure. For mere mortals like us, I have found it beneficial to start with half notes. If you have a ii-V-I in C, with a measure of each, just play the 7th and 3rd over each chord. Those are the "guide tones". No problem with a root or 5th in there either. The trick is to get those chord tone sounds in your ear so you can hear them against the harmony. Once you get that, you can move to quarter notes and approach the chord tone by a diatonic step either below or above the target. You will be surprised at how good it sounds. Clifford Brown made a living out of diatonic lines that just employed this type of thing. At this point you have quarter notes, so you double it and get your eighth note lines. This is where you experiment, keep trying to lay the 3rd's and 7th's on the 1 and 3. But try approaching chromatically from above or below (or both). Or if you are moving a whole step to get to your target, go ahead and include the passing tone in between. Barry hits on this stuff occasionally, the above/below/target thing is generally referred to as "enclosure". Bottom line is that I've found that if my lines suck over a progression, I back out to the 3rd's and 7th's which will always work and then build on them. Good Luck!