Never underestimate the value of doing a little analysis. I say a little because a limited amount of calculation puts you a surprisingly long way ahead of those that don't, or can't be bothered. And I say a little because if you think carefully about what to calculate it limits what you have to do, to just what is important.
And remember the assumptions you make. Write them down, let people know. Often the thinking that sets the assumptions is more important than the calcs. They set out the quality of your thinking, they prompt a debate.
Decide what it's important to calculate
Think about what assumptions to make and write them down
Do the simple calcs and draw the conclusions.