Hi,
The AD9912 datasheet says: FTW = 2^48 * (fdds / 1GHz) where fdds is the desired frequency and 1GHz is the reference clock.
As per the Wiki example for the AD9850, but with these numbers instead, you calculate 2^48/1GHz=281475 which is what I believe is the "magic number" you're referring to.

So, FTW = fdds * 281475 which we can verify against the example in the datasheet:
19,440,000 * 281475 = 5471874000000 which in reality would result in a frequency 1.6085Hz higher than ideal due to 2^48/1GHz not being exactly 281475. Does it matter? That's up to you.

Now, LONGS are "only" 32 bits so what's the highest frequency possible? Well, 2^32/281475=15258.788Hz.

Since the ** operator, when used with LONGs, results in an 48bit wide intermediate result I would guess we could use some trickery and retrieve all 48 bits directly but I don't have time to play around with that right now.

/Henrik.