Dear AnnMarie,
Thank's for the quick and valiant effort to answer this question.
Unfortunately I am still none the wiser as the 1:3 ratio rule is well known.
If this formula is correct then a 1 sq mm inlet would suffice as long as the outlet was 3 sq mm. Clearly this is not the case.
It would appear that US Engines has not established a minimum inlet area and I'd be surprised if any other maker has either.
If I can establish this data somehow, I'll be sure to pass the info on to you.
Many thanks,
Merlin