This post was written by David Walker on the old CUSF website.
It is important that vents are drilled in the rocket so that the barometer on the flight computer can sample the atmospheric pressure (and calculate the altitude). There are various rules of thumb for sizing such ports but few explanations from first principles. Intuitively (or based on dimensional analysis if you will) there must be a ratio between the volume to be vented and the area of the vents. One rule of thumb is one 1/4” diameter hole for every 100 cubic inches. An approach based on first principles goes like this.
First decide on the maximum allowable error in the altitude, say 10m. This provides the pressure difference between the altimeter bay and the free stream. Next find the maximum flight speed, this gives the maximum rate of change of pressure. Modelling the altimeter bay as a volume V with an orifice area A and pressure drop delta p this gives all the information needed based on Bernoulli's equation for the flow through the orifice and mass conservation. This Mathcad sheet calculates the size of the vent holes. (Mathcad Prime Express can be download for free).
Based on an error of 10m and a flight velocity of Mach 2 the calculation yields a 0.176" hole for every 100 cubic inches, a fair bit less than the rule of thumb above.