Something I've done that helps following floor plans is to draw it out first, I usually use grid paper. The reason I draw it out first on paper like this is so I can visualize it and get the dimensions correct for the grid in build mode, because you're right, eyeballing it can be difficult and it doesn't always come out correct! Sketching dimensions on grid paper will help you see how close to the original you can make it while also seeing how big your rooms will actually be. :)