New question: Credit score and credit card usage ratio

[Beartracks]

I've always heard you need to keep your credit card usage below 30% of card capacity. But is that the rule for each individual card, or is it more important that all your credit card balances added together add up to less than 30% of the credit capacity of all the credit cards collectively?

And here's why I ask.

I have an opportunity to take advantage of a 21-month 0% transfer promo on one of my cards, and there's enough room to transfer almost all my other credit card balances onto it. That's going to max out that one card, while zeroing out (or very nearly so) all of the others. But my total usage among all the cards together will remain about 28%. Will maxing out that one card cause a hit on my credit score even if total usage doesn't increase, and stays around 30%?