I wouldn't worry about that; it's really hard to screw up making a water block. However...
I'd say "look at other designs", but the cross drilled one is pretty rare, because it take a long time to make. We haven't seen your design yet; I'm assuming that you're cross drilling a whole bunch of small holes (run a search here for a cross drilled block, someone made one)?
Otherwise, be aware of the thickness of your baseplate, where the "baseplate" is the thickness of the copper, before it touches the water. It shouldn't be thicker than 1/4 inch. (btw, the math part is where you calculate the ideal thickness).
If you're not sure about your design, you can create a heatload tester (power resistor) and take some temp measurements. Alternatively, someone here might volunteer to test it, if you ask nicely. Otherwise, I'd say try it on, but keep the computer idling (no CPU load test) until you have a good idea of wether your block is "in the ballpark" or "out to left field".
Your results are also going to depend on the coolant flow rate (determined by the pump) and your cooling solution (radiator-fan combinaion).
Let us know if you have any more questions, and if you have a chance, please post a picture or diagram.