To determine which logical tests scenarios are eligible to be automated you can ask yourself the following questions.
1. Is this or part of the scenario already covered by a system integration or unit test?
2. Can the functionality be tested with a more ‘low-maintainable’ test?
If I have a functional test scenario I try to put it as low as possible in the testing pyramid. If the level is not suitable for the scenario I go one level up. Think about a situation here were you want to automate the following scenario. A user enters an account number into a web application form. The server can reply with a valid or invalid response. The invalid response can result in 20 different error messages. Given this situation a tester can write functional scenarios for all error messages. This can result in possibly 20 functional testscripts. My solution for this situation would be to verify the server frontend mechanism with one functional automated test and verify if all the messages are returned correctly with one or more backend integration testcases.
3. Is this critical system functionality?
This question is especially for starting projects, when you start writing some script, take baby steps. Think, “what is the minimalistic thing I want right away!”. Keep building on top of it whenever tasks get complicated. Functional testing scenarios often have a lot of overlap in test steps to be able to prepare certain scenarios. It will save you a lot of time in the future when generic functions are created which can be reused in the test scripts. I see the generic functions as lego bricks which can be used for automated testcase construction.
Test automation is becoming more and more important due to faster development iterations and agile methodologies. Overthinking were and how certain functionality of your application will be tested can take effort to accomplish but it saves you a lot of valuable work time in the future.