this will depend greatly on who you ask, what oil you will run etc. Some people will recommend building them tight, near 1 thou on the mains, others will say 2 thou on the mains, general rule is 1 thou per inch of journal, and the more power you wake the larger the clearance. Personally i have built some sr20s loose, 2+ thou on the mains and was fine. I don't think there is the perfect rule, but building them a little on the looser side makes some sense to me.