Good reason is intentionally left a bit open for each person to discern. For example, being in an area where one has to travel is a good reason to some and not others.
As long as good reason is not “dumbed down” to a level of scandal one needs to trust their conscience and advice from their spiritual advisor.
I am pretty hard on myself for what is a “good reason” to miss mass, but would not have as high expectations for others.
I have confessed missing Mass because I was camping at a motor race and there is no Catholic service available (usually just a non-denominational service); I can’t drive, so I can’t leave the grounds. Better safe than sorry, I figured. However, since I used to work for a newspaper and they NEVER close, almost all the weenie excuses are excluded. Once I went to work on skates because the streets were iced over. If I can go to work on skates, I can go to Mass on skis!