You don't say how much of a CL you have on Discover to accomodate the Chase balance. Depending on the balance you could take a hit on utilization for the card, but if you're not applying for anything and just want to pay down faster it can be OK. I wouldn't want to BT to a card with a balance since any payment will usually go to the highest interest balance first. Although it depends on how much you are paying toward the balance. Selecting the particular promo depends on how long it will take you to pay down the debt. I'd suggest to find a payback calculator, particularly a snowball option type, online and input your data and look at different options.
The percentage of a card's balance relative to its limit is called utilization. The term is also applied to total balances relative to total limits. You can refer to individal card utilization or aggregate utilization.
"Below 30%" means to use 28.9% of a card's limit or below, or in aggregate, to use 28.9% or less of your total available credit limits. This is the threshold that's considered responsible borrowing. The reason for 28.9% is that 29.0000001% rounds up to 30, and you're no longer below that threshold.
8.9% or below is even better as that optimizes scoring.
Having less than half of your cards reporting balances is also ideal. Of course, that's one of the things you're trying to fix. 
It's not as important as utilization, though.