As a Civil Engineer I did plenty of force calculation scenarios in my career so I can help.
You need to account for all the worst case scenarios you might encounter and then design for those and add safety factors.
Then its a calculator festival to calculate all the forces and strength of materials to finally arrive at some concrete numbers.
What implement do you want to start with?
We can go through it together here for all to learn.
I'm no tractor expert but many others here can surely think of worse case scenarios that we did not think of.
Just some things to consider, for safety reasons you don't necessarily want your implement to be the strongest possible thing on your tractor.
For example, do you want your loader bucket to be so strongly (heavily) built so you could take out a bank safe, but it will be so heavy your rear wheels
will come off the ground and spin uselessly. Do you add more rear weight to correct for that, and then find your front axles failing under load.
Make one thing stronger and something else will break, you have to choose what you want failing first to save the more expensive parts.
Its all compromises so you have to design accordingly.
Now for those that are not engineering types, the "looks about right" trial and error based on experience, is a legit method as well.
That's why you need to be a good welder, if its too weak you can always add some reinforcement.
You just have to be extra cautious and expect your home made stuff to fail until you have proven its up to the task.
