I don't think there's a simple formula to convert scrypt hashpower into groestl hashpower, a 7950 and a 5970 have almost the same scrypt hashrate (700-800), but a 7950 has around 6000 KH/s in groestl, while the 5970 has more than double. But I guess using something like grs = scrypt * 12 is close enough for most cards...