As has been pointed out before, by reusing the whole yeast cake you will almost certainly be overpitching.
Some beer styles (most notably hefeweizens, but also some English styles and others) gain a lot of their flavour from compounds produced during the yeast growth phase. If you're concerned about this, you can figure out the appropriate pitching rate using this pitching rate calculator
and just dump out the appropriate amount of the remaining yeast cake.
That said, I pour new wort onto a whole old yeast cake all the time and have never got anything but a good beer out of it