Hopefully you havn't drilled that hole yet as there is another thing you should look at.
Considering there is no baffle under the valve cover, you want to drill the hole so it will be directly between any two rockers. This way when you look in the hole, you shouldn't be seeing the direct tops of the rocker arms.
The reason for this is because oil is sent to the rocker arm through the pushrod which than can fly upwards. If the hole is directly above the pushrods, oil can very easily go through the hole and through whatever breather system you have hooked up. Now if the hole is between the rockers, oil will just bounce off the valvecovers and splash back down.
This is a long post about this but it should be taken into consideration as it will knock down the chance of oil coming out about 95%.
Measure twice, cut once.