how to understand a positiveresist PCB developed enough
What you say sounds much like you overexposed the photoresist - that is: You had the UV light on for too long. 5 min sounds like a lot (but it's been a while since I did my own PCBs). If I remember right, I used something like 0.5 to 2 minutes, but this of course depends on the type of lamp you use.

The right exposure time is way more critical than the right time in the developer.

Also, the concentration of the NaOH does matter: How small was your small cup?

A bit less likeley, but also a possible cause: The contrast of your film was too weak - that is: The black stuff wasn't black enough.

As a general guideline: I had to do a lot of trials (and errors!) until I knew all the specifics, and they are plentiful: Exposure time (much depending on the "strength" of the UV light), concentration of the developer, temperature of the developer, developing time, ...

Here's a good related answer (although it is about underexposure): https://electronics.stackexchange.com/a/63989/930