Yes, I think that point needs to be made more often, that the famous Drake Equation is not an equation at all, and should be more properly called the Drake Checklist. When we know all the numbers, when we have values for things like ‘what percentage of planets have evolved life, then we will probably have already established contact and tourists from Earth will be traveling far and wide throughout the galaxy, and annoying the hell out of everybody.
But I think the stickler might be ‘percentage of planets where life evolves which evolve technological civilizations.’ Perhaps I’m thinking a bit too much of ‘life as we know it, but it seems to me two things are necessary for that – opposable thumbs and the ability to work metal. And that means any intelligent life that evolves in an ocean is probably out of luck, because you need fire, and any airborne creatures wouldn’t be able to accumulate the raw resources, so you need to evolve on a planet which has plenty of water, and an atmosphere, but also enough land to support a technological civilization. Seems to me, it might be a rare combination.