Different brands and component levels of levers have different shapes, and different bars have different amounts of drop and different curvatures. Arne's suggestion of lining the tips of the brake levers to be even with the flat part of the drops is a common rule of thumb to get you started. If the bars are already installed on the bike you can do this with a ruler or something else flat and rigid. I found a decent image illustrating just how to do this, albeit on an older bike:
Notice that the owner has left a quarter inch gap above level. This is where the ergonomics of the components you are running and your own personal preference come into play. If you are unsure, I would start with this basic rule of thumb and work from there. A few things to note:
- moving the levers too far up or down on the bars is going to give you weird ergonomics in terms of hand positions and lever reach from the drops
- once you have the levers set where you want them, be careful of twisting the bars too far up or down in the stem as this will also give you weird ergonomics. On newer setups you typically want to start with the transition from the bars to the hoods being pretty much parallel with the ground
- once the tape is installed on the bars you only have so much wiggle room to slide the levers up or down. Ultimately if you feel like you can't move them enough with the tape in place then you're probably trying to make too big of a change- unless you started with them clamped way too low or way too high on the bars. A little change can make a big difference, so make small adjustments and then go for a good ride to try it out.