Verification of approach to calculating power dissipation, temperature and use of heatsink

There is term missing from datasheets, which is the thermal resistance from case to heatsink.  The case-to-heatsink term is affected by how much contact area they have, which depends on how flat the surfaces are, which is not controlled by any single datasheet, so it isn't often listed, but you can look it up for various materials. This resistance term can be reduced by themal compound because although thermal compound has themal resistance it is lower than the thermal resistance of the tiny air gaps between case and heatsink.

The heatsink-to-air term is only accurate under specific conditions, because it is significantly affected by airflow and dust and humidity, and airflow can be caused by anything, including the hot air rising off the heatsink. And of course the ambient air temperature is constantly changing as the heat coming off the heatsink changes it. And then there are external heat sources to consider, like sunshine, that can add 1KW per square meter. It sounds like you know all this stuff, but it is mentioned just in case it is useful....

When you need to squeeze the last bit of cost and performance out of a system there are a a lot of factors to calculate, including how long the system needs to operate because even moderate temperatures will reduce lifespan. I generally do rough calculations without thermal paste for a temperature target well below maximums listed in datasheets and add thermal paste to provide extra margin.

I'm pretty much aware that there's a lot that I don't have experience with yet, that's for sure, but intuition tells me that ambient isn't likely to stick at 25c as things warm up!  I do suspect better understanding comes with time and experience but I have to start somewhere so if I'm on the right lines then I've got a good point to start from.  From the calculations I've made, based on my understanding above, I've selected heatsinks that keep temperatures well under maximums (around 60% at worse case) as a tradeoff between cooling vs design size.

Interestingly, the one part that may give me some grief is a MOSFET but its data sheet does list Case-to-Heatsink which I've included in my calculation!  Modelling with LTSpice shows that it may be dissipating up to 12W worse case, depending upon usage (power supply), so needs a good heatsink.  This is the one part I'll need to carefully monitor when testing as that worse case calculation gives a temperature of 80% of maximum - typically, I think it will be a lot less.

I'm pretty much aware that there's a lot that I don't have experience with yet, that's for sure, but intuition tells me that ambient isn't likely to stick at 25c as things warm up!  I do suspect better understanding comes with time and experience but I have to start somewhere so if I'm on the right lines then I've got a good point to start from.  From the calculations I've made, based on my understanding above, I've selected heatsinks that keep temperatures well under maximums (around 60% at worse case) as a tradeoff between cooling vs design size.

Interestingly, the one part that may give me some grief is a MOSFET but its data sheet does list Case-to-Heatsink which I've included in my calculation!  Modelling with LTSpice shows that it may be dissipating up to 12W worse case, depending upon usage (power supply), so needs a good heatsink.  This is the one part I'll need to carefully monitor when testing as that worse case calculation gives a temperature of 80% of maximum - typically, I think it will be a lot less.

One other factor that often comes into play is if any parts may come into contact with people. Anything over 55C is considered too hot to touch (by me, but choose your own people-safe temperature).