3 is close if it's your first day and you factor in the 200 confirmations.
5.1/total_hash * XVC/block * 18 * 24 = XVC/day
So, assuming 510 GH/s total hash, it would be
5.1/510 * 1.027 * 18 * 24 = 4.43664
If there is only 255 GH/s on the coin, then you'd get 8.87328 per day. Difficulty jumps around a bit so you might have to look at it a few times to get a sense of average difficulty. Just seeing what shows up in another 24 hours might be easier.