Bidding accurately in JJDD is not intuition. It's counting. Every hand has a number you can arrive at by sorting your cards into three buckets — Guaranteed, Likely, and Possible — and doing simple math on the result.
Guaranteed books
A Guaranteed book is a card nothing can beat: the Big Joker, the Little Joker, the 2♦, the 2♠. That's it. Four cards in the deck qualify. Any of them in your hand is one book you can bid without hesitation.
That's the entire Guaranteed tier. The Ace of Spades is not in it — the four cards above it can all take it.
Likely books
Likely books are cards that win most of the time given normal card distribution. The Ace of Spades is the biggest one — beaten only by the four Guaranteed cards, so it takes a book about 4 out of 5 hands.
The King of Spades is Likely if you also have the Ace or a Guaranteed. Aces of non-trump suits are Likely if you're not void everywhere else — because they can be cut, but usually won't be on the first round.
Count Likely books as full books when you have two or fewer. When you have more, discount them: three Likelys is worth about 2 books, four is worth about 3.
Possible books
Possible books are cards that might win depending on how the hand plays: a King with no Ace behind it, a suit where you're short and might be able to cut, a spade in the middle of your trump line that could take a book late if the top spades get burned.
Don't add a full book for a Possible. Add half a book — and only if you have two or three of them combined. Overcounting Possibles is where bad bids come from.
The math
Add up: Guaranteed + Likely (discounted for quantity) + half of your Possibles. Round down when in doubt. That's your bid.
Then look at your partner. If they've bid before you, factor in what they might have. If they've bid after you, expect them to cover the four-book minimum and don't inflate your number to help.
