How to determine the ideal height stool for a given height surface (table, counter, bar, etc.)

There is a large body of information about what size furniture and tools should be. The field concerning sizing furniture to individuals and populations is called Anthropometrics and information about safety and ease of use is called Ergonomics.

If you are building your own workbench for your own use, then you should size it to yourself unless you expect others to use it. Why buy off the rack if you can get tailored for less?

Most furniture is optimized for the middle of the height distribution curve which is roughly 5'8". I got into woodworking in the first place because I am 6'4" tall and absolutely nothing fits. It's all literally 6" too short.

I used the "fall drop" rule to size my generic workshop surfaces. Just stand relaxed and let your hands fall down as if resting comfortably on a surface in front of you. It's the position you hands will just fall or drop to if you try to rest them on an imaginary surface. It gives a higher bench than the using the wrist line method above.

I suggest creating a mock-up first using whatever is available and easily resized. I used a chair, a sturdy box and books of various thicknesses. I swapped books and fiddled with the height of the mock-up until I found the most comfortable height. In my case, it was exactly 39".

You'll know it when you find your own ideal height because it just feels right.

However, shuffler is correct that different work requires different heights. The "fall drop" rule sizes a bench for use with modern power tools and precision hand tools but if you use other tools you might want a higher or lower bench e.g. heavy handsaws, chisels and planes usually require something like saw bench which is usually just under knee height.

I use a mini-bench on top of my main 39" surface if I have fiddling work like soldering.

Oh, and build yourself a custom stool and work supports at the same time. You'll be glad you did.

I am guessing that your sink weight is causing the pressure, making the cracks worse. If I am right on this then the first thing you need to do is build a simple way to support the sink using wood or whatever - I have 2x2s that are screwed to side cabinet and pushed up to support it. So that will cost you $3. If side walls are not close enough you have to build a box.

Now to fix the cracks you will need a knife grade epoxy. I have used Tenax brand in the past to "glue" granite corner shelves in the shower. Pretty easy to use. Epoxy will be about $20-$30.

And for some reason (conspiracy) the big boxes don't carry any knife grade epoxy. You will have to hit the interwebs for this. Just search for knife grade granite epoxy if you want to read reviews or whatever. Basically you will get the activator and the putty. You put a little activator in the putty and wipe it on the granite. If you are doing the top then you might need to by color matching kit (which is more expensive than epoxy). Basically it is hard in 30 mins and can be used the next day. Once epoxy is on it is permanent and harder than the granite. If you arent worried about looks your sink area could be cured of the crack as long as you get the epoxy in the right places.