The hard part was figuring out?

1) The hard part was figuring out? What not "The hard part was figured out"? Because only humans have a mind and can figure. Others? No.
2) What does "The rest was calculus" mean? Does it mean "the rest was trivial"?

Context:

During the Cold War, scientists modeled almost every imaginable consequence of a nuclear explosion. But Dillon found a gap in the sheltering strategies for people far enough from ground zero to survive the initial blast but close enough to face deadly fallout. He focused on a single low-yield nuclear detonation like those that destroyed Hiroshima and Nagasaki. The world’s nuclear arsenal has grown far more powerful—today’s warheads can inflict thousands of times more damage—but security experts believe that low-yield bombs are the kind most likely to be used by terrorists.

The hard part was figuring out what variables matter for fallout survival. The rest was calculus. The longer you stay outside, the higher your radiation dose, but the environmental radiation intensity also decreases over time. So your total dose is a function of when you step outside, your distance from the detonation, how long you run before you reach better shelter, and how much shielding you get from the local environment while you’re out there. Dillon simplified the calculation by assuming that you are totally exposed while running to safer shelter; he also ignored complexities such as limited shelter capacities. In the end, the math boiled down to a single critical number: the ratio of the time you spend hunkering down in your first shelter to the time you spend moving to the high-quality shelter. Then Dillon worked out what would happen with a variety of shelter options and transit times.

More:
http://news.sciencemag.org/environment/2014/01/how-survive-nuclear-explosion