What I want to do is go to the Tools->Boards menu and have an entry called "AtTiny85 1MHz" and have aa definition of F_CPU=1000000. The advantage here is that I can use F_CPU to figure out what clock prescale to set, and by having this set for the board it will force included libraries to compile with F_CPU=1000000, which means that libraries like millis(), that rely on F_CPU, will be compiled with the desired F_CPU. But between boards.txt, the hardware subdirectory, and what appears to be a convoluted and undocumented directory structure, I haven't a clue as to where to begin.
I would like to write my source code so I can determine the desired clock speed by selecting a board with 1, 2, 4, 8 or 16MHz in its name, and the code will compile, select the right prescale in setup, and just run my code. So if I have problems at 1MHz then I can try 2MHz, rhen 4MHz, as I look for a speed compatible with my external circuitry.
The goal is to produce a really-low-power system that can run off a coin cell battery. It will also be asleep 99 44/100% of the time, which makes for good floats. Or is that soap that has floating point? Anyway, I hope you can see what I'm asking for and suggest ways to create a new board type. I couldn't find an old board type for AtTiny85. But I might be looking in the wrong place.