I've always drilled a hole from each end of the eventual slot straight out at 90°, unsure why you say you may end up coming through somewhere you shouldn't but hey ho. I'd then go outside and strike a line between both holes which should be level and then drill from the outside in but only through the first chamber, then from inside I'd finish of between the 2 holes going through all the chambers bar the last, my reasoning is if you drilled every hole from the inside and straight out you'd probably find none of your holes outside were level and would probably look like you've used a machine gun to put them in, probably find that the trickle vent wouldn't cover the holes as well or if so wouldn't be level.
Of course you then have reinforcing to contend with if your unlucky, such a simple looking job Saturday morning job can quickly escalate into a whole weekend