You should use resistors that you are using in your application. Your differential amplifier will work in a specific configuration, for example with the feedback path as you draw, is that right? If so, just use these value of resistors. As a results you will have a real value of slew-rate. Not the case where you have one value of SR in a SR testbench and another one in TOP simulation or somewhere else.
Resistors in the feedback paths would rather not affect SR value so much unless they have low value comparing to the current possibilities you have at the output.
You measure SR as you wrote. Using calculator. Remember as written here in a paid version
https://payhip.com/b/5Srt, but also other books or in the internet that you have different SR values for rising and falling slopes. Depends on the opamp architecture.