imobilinpedalus ,
Let us come at your question from a different direction.
Different people like to operate a different RPM's. My buddy like the mid 60's RPM, I like 85-95 RPM.
Different people like (or are able) to travel at different speeds.
Different people like to operate on different types of terrain.
Different Derailleurs will only permit certain gear to be used. Often limiting not only the size of the largest cog on the rear, but limiting the difference in size between the largest and smallest cogs.
Meaning that even if you can calculate a gear you wish to use, there are a whole set of manufacturers rules regarding IF you can even use the size you wish with the bike you have.
So the question you have asked, has too many variables for us to give you an accurate answer.
Most people come at your question differently.
My example from some time ago.
I've have a Cervelo R3 currently fitted with Crankset FC-R8000 50/34 chainrings (front) and CS-R8000 11-30 cassette (Rear).
The derailleurs are (front) FD-R8000-F & (rear) RD-R8000-GS
I climb a lot of hills and require easier gearing, what can be done?
ANSWER: Change the cassette for a CS-HG800-11 11-34
Don't forget it will require a longer chain to be sized and installed, plus full derailleur adjustment including "B" screw.
I've taken all of the human element out of the question, I have detailed the existing equipment and the issue I am having with it.
I wish you all the best and hope you decide to alter your question to provide more details.
Barry