Here's how to get a close as you can to making circles and domes in minecraft, and remember, the bigger the dome, the better it will look.
Type in the equation x^2+y^2=r^2 where r is the radius you want your dome/circle to be. Make sure the graph setting is on implicit graph, not f(x) or polar modes.
Look at the upper right quarter of the circle, look at the squares where the center of the square is in or on (or very very close) to the line, that will be the boundaries of your circle.
Copy the part of the circle with blocks, repeat with the other 4 parts until you have a circle
Now the side of the dome has the same dimension as the base, so you want to copy the circle shape vertically.
Then type in he equation for the next radius in the vertical stacking of circles, in this case it's 5
Look at where the squares are located in relation to the circle and do the same process as before, just on top of the other circle
If you get this occurrence, fill it in the way you think looks best
I like to put one below personally
Here's the 5 radius circle
Now do the next circle size, which for this is 4
Repeat the process
And you get this
Now for the last one, which is radius of 3
Fill I'm the top, and there you go! As close as one can get to a dome
The domes look better and better the bigger you make them, so the smaller ones don't look very good :/
Need a guide on minecraft? Message me what you need and I'll make a guide for you! Also check out my YouTube channel- www.youtube.com/sircookthebook