I never go by speed or rpm when shifting (probably because my first manual car did not have a tach.). Always by feel. If you feel the engine is running too fast, upshift. If you feel it's running too slow and struggling downshift.

I believe it's TJ that said in a previous post: To achieve best fuel economy, drive in the gear that will make you press on the gas pedal the least.

If you upshift too quick, you will have to compensate by giving it more gas to keep up.

If you don't upshift, you will also have to give it more gas to get enough RPM to drive the desired speed.

There is no magic numbers. The best thing is learn to feel how the engine is doing and adjust the gears accordingly, so that man and machine becomes one.