Capricious,
That's right. A heatsink that is rated at 2.4 K/W increases its temp 2.4 degrees Kelvin for each Watt that it must dissipate, in standing calm air. This is called its thermal resistance. You use this value to determine if the heatsink is large enough or whether it will need forced air cooling with a fan. I guess they use the term "Kelvin" because they don't know what your ambient temp will be. Many parts are spec'd with degrees Celcius/W.
There are additional thermal resistances that must be accounted for:
1) Between a transistor's junction and its case.
2) Between the transistor's case and the heatsink. Because the surfaces are not completely flat, heatsink compound grease conducts heat and helps to fill the small voids. But it must be a very thin layer.
3) An insulator also has a thermal resistance rating.
4) The amount of mechanical pressure between the transistor's case and the heatsink. Many "clips" fasteners do a poor job compared to bolts and nuts.
If your heatsink requirement caculates as being too large, or you don't want to use a fan, then add parallel output transistors (with emitter resistors to equalize their gain differences) so that they can share the current and heating. The driver transistor will also run cooler since a power transistor's gain decreases when its current increases. So 2 paralleled transistors have more gain than 1.
