1 and 2 foot levels are not the tools for longer runs. When doing fence posts or door frames you need a 6' level.

As for the Pythagorean theorem? I use it all the time as a reno carpenter and works great for me when need be. And I failed math.

Most people that tackle DIY think carpentry is easy and why do they charge?

Well we charge like any other trade. Years of experience and thousands in tools. All the right tools. You can not use a small level for door installs or fence posts.

Don't get me wrong. I'm all for DIY and learning but it comes at a cost. The cost is the learning curve and more important the right tools.

This shit was not that easy to me 40 years ago but dead simple now. I do without thinking.

Keep at it. You will get better. But you need the right tools. And this is what a lot DiYers don't get. You can not do a good build without the right tools. So you need to spend money to save money. And practice. Keep the renos simple and learn as you go.