You are right, kxenos, the 2nd formula IS wrong.
Rja = Rjc + Rcs + Rsa
The spec sheet should give values for Rjc and Rja. The Rja value assumes no added heat sink, so the case IS the heat sink, and Rja = Rjc + Rca
jc - junction to case
ja - junction to ambient
ca - case to ambient
cs - case to sink
sa - sink to ambient
So for a TO220 regulator that specifies 3 d/w Rjc and 75 d/w Rja, without a heat sink, 1 Watt would generate a 75 degree rise above ambient. With a 5.5 d/w heat sink, you would have
3 d/w Rjc
.5 d/w Rcs (approximately)
5.5 d/w Rca
9 d/w Rja, for a 9 degree rise at 1 Watt.