Derate heatsink when used passively
It is a challenging problem to de-rate a heat sink. The problem is that if a heat sink is designed for forced air cooling (using a fan), the spacing between fins is optimized for forced air, and usually is fairly dense, 1.5-2mm apart. For cooling with free convection however, the fin spacing must be at least 1/4" (6mm) or more, for the ambient air to flow between fins with any reasonable velocity, and for thermal boundary layers not to overlap along the fins. Needless to say, you should maintain vertical orientation of cooling surface and vertical orientation of fins to have any positive effect.

In other words, forced-air heat sinks are usually unsuitable for free convection cooling. The best safe estimate would be to consider the entire heat sink as a brick.