You can start with one 3/32 hole.
Assuming you do not have a restrictor valve and are pumping straight to pipe, your using the hole as a restrictor. For testing purposes in a 4 banger your looking for ~1/2 gal/min flow. Not exacly sure what your fitting looks like but you'll probably get to 2 holes at 3/32, or one 1/8. You will end up with an exit area ~0.012"-.014".
On my home made system we started out with 3 no.50 drill size holes. That worked well but we ended up with 1/8 inch hole. 2 injectors one each in 3 and 4 pipes of 4 pipe system. It will put out more oil than it can burn without a flow restrictor. I have found that the different types of smoke oil vary quite alot in the amount and quality of smoke. When you are testing your system make sure you use the good stuff. The best I have found is the Super-Dry.
Smoke Oil Orifice
Linear Mode
