Almost certainly ( at least for any decent manufacturer ) there will be recommended solder paste patterns for a package in datasheets etc. The rule of thumb would be to use those.
Looks like Atmel might not be one of those decent manufacturers...
Or I've been doing a terrible job of searching for them. But I've also been looking at various power LED packages from smaller manufacturers, and.... it's messy. There's a 2835 package that appears to be an industry standard, but there seem to be rather different PCB footprints between manufacturers!
evb149, thanks for the list! That's going to take me some time to get through though. 70% is rather small though, not to mention 50%! I was guessing 80% but now that seems way too much.
I've been using EAGLE thus far, and IIRC the paste layer is automatically shrunk a certain amount relative to the copper layer, although it's possible to delete it and insert one's own during footprint creation. I need to check that. (I also need to start using CircuitStudio, but that's a different matter entirely...)
I'm not doing any commercial manufacturing with this, and am probably going to go with OSH Stencils for my first time round.