[antonylee]

After 1m-1M; 1N-2D! (GF checkback), there is usually plenty of room available, e.g.
1♣-1♠
1N-2♦!
2♥-2♠ (4 hearts, maybe 3 spades (let's say you respond up the line); 5 spades, no heart fit)... now you have everything from 2N to 3N available (and even all the way to 4♠ when holding 3♠).
Recently we had a hand that led us to add the agreement that in this auction, a jump to 3N over 2♠ shows a fit but with a NT-oriented hand, but this is pretty ad-hoc.
Any thoughts on how to best exploit the available space? (Things applicable in an unbalanced diamond context for 1♣(2+)-1M; 1N-2♦ would be appreciated too.)

[KingCovert]

So, let's break this down, you have a 12-14 balanced hand that has more clubs than diamonds (or 3-3), exactly 4♥, and either 2 or 3♠. So, your possible shapes are:

3433
3424
2434
2425
1435

So, if 3♣ shows exactly 5♣ which guarantees no 3-card support for spades. I'm going to assume you wouldn't rebid 1NT with 1426 shape... Seems a safe assumption. Then what shapes remain? 3433,3424,2434. I wouldn't rebid 1NT with 3415 shape, so, it's eliminated. All of these shapes are No-Trump oriented, so I guess the real question is, are there enough hands where you think the distribution of your values when you have completely flat shape like this is significant enough in your mind to have a clear preference for 4♠ over 3NT? You can make this agreement, but, it seems a rather useless one. 3♠ says exactly what your 3NT bid says sort of by default. It's really not up to your hand what the contract is.

3NT probably should just be to play, and if it is, what is 2NT? Why doesn't 2NT mean what your 3NT bid has been agreed to mean? On the hands where responder has slam interest, your agreement for 3NT seems like a rather huge waste of bidding room. Don't consume bidding room agreeing fits in constructive auctions, it's sort of self-defeating.