Suppose you have 50 test cases for which you need to do automation.
By your own wisdom or maturity, you have decided that 20 can be
automated and the other 30 have to be manual. How would you deicide as
to which type of test cases has to be manual and which can be
If you have 50 test cases that require automation, then decide on the more functional aspect of test cases that require automation. Leave all the User Interface test cases to be manually tested and the core functions to be automated. This way, it reduces time and also the test scripts ensure that the functionality of the system is as intended.
Adding to what Justin said, It may be easier to automate only part of the existing test case, e.g. you may automate all 50 TC but only for some 40%, leaving 50 smaller test cases, or at least leaving 50 test cases that have less possibility to fail.
For example you may be able to automate server part of functionality and leave client (UI) to test manually.