Simon,
I'm sure many people are going to be thinking this, but it shouldn't really be about a formula, but what fits best to the land.
Having said that, most of the courses I grew up playing in Derbyshire were over some pretty wild terrain so there was no end of interesting par 3s and short par 4s, as these can probably cope with the difficulties of the existing land. Now whether its for that reason or some other, but I actually like to see plenty of par 3s (even though two of my favourite courses of Hollinwell and Deal, both only have 3). And as has been suggested from some of the par 5 discussions recently, they can be tricky to fit in or design well, so this woul hint at fewer of them if possible.
One of the Derbyshire courses that is still a big favourite of mine is MacKenzie's Cavendish which has five par 3s and one par 5 to give a par of 68. That sits pretty well for me. I suppose its also worth mentioning The Berkshire Red with its 6, 6, 6 combination, though it should be pointed out that at least four of the par 5s are more like long par 4s by todays standards, which would again give it a par of 68.
Bearing all those in mind, if it fitted with the land I suppose I'd quite like the five par 3s and three par 5s that Ally mentions.
Lastly, I've never really understood the par 72 obsession? Other than it allowing Alliss to mention how someone is doing in relation to "level fours" where did the obsession come from?
Cheers,
James