Might as well go every two feet on the floor joist. You could go 2 1/2 feet but you not saving many boards. I would go with 2x6 on the floor. They are less expensive than the deck boards. I wouldn't leave a crack when putting the floor down the boards are going to shrink quite a bit when they dry out.
depending on the weather/humidity where you are...I know in central florida you better leave a gap, they will expand and contract.
I agree with ETH on not bothering going to 30"...you can do the math, you wont save much...unless you're building upwards of 20 kennels
