I find the Kaplan scores are about 100 points below my scores on Powerprep and the Manhattan Practice tests.

I haven't decided what the best spacing is for taking practice test. I figure you take them when you feel you have improved. I think of the practice tests as checkpoints, but also as confidence boosters. If you know you are not ready to pass through a score target, do more practice problems and then take your full length test when you are better prepared. You want to leave each practice test feeling like your work has paid off.

I have taken a few practice tests late at night and consequently performed poorly on the Verbal section. I was too tired to concentrate. Better to put the test off to the next day than to hurt your ego with a bad score, when you could have done better.

That's my thought.