What is the best way to determine the ideal shape of the timing vs rpm curve?  

Umm... There is a highly scientific method. It's called guess and check...

Some large cammed engines will like a lot of total brought in fast. Think about drag engines with locked distributors. But, this can also cause mid-range preignition and then has to be tuned out.

The rate at which an engine will like the rate of mechanical advance is dependent on many things, such as: combustion chamber design, camshaft design/overlap, fuel octane rating, stall speed, compression ratio, etcetera, etcetera. Just as long you have sufficient initial timing and the right total coming in a decent time then I think the rate will take care of itself. I think the "industry standard" is "all in" at 3000 rpm. Maybe if you have a slight traction problem you can bring it slower, or like I said before, a mid-range pinging issue you can bring it in slower.  

Crane, I believe, makes an adjustable vacuum advance diaphram that comes with pretty good instructions on how to set both vacuum and mechanical advance.  It works quite well.