Homebrew PCBs

I just made my first PCB. I did two of them, with different expossure
times.

Two 6W blacklight tubes, 7 cm away from the PCB, trough a 0.5mm glass.

First, I did 5 minutes exposure. Then, 2 minutes 30 seconds in
metasilicate developer. It came out quite well. All shiny. But it
seems that when I took it out of the developer, and tried to clean
with a soft dumped paper, some of the etch-resistant tended to difuse.
So I stoped, then washed it with lots of fresh water, and then dry it
with some toilet paper.

It came out well, but some borders not perfectly defined. So, I tried
more expossure.
This time 7 minutes, 30 seconds, and 2 minutes in metasilicate.
This one came out better defined, but tracks were thiner, and a little
section of a thin track dissapear.

These are my conclusions...
the second board was overexpossed. I should try near 5 minutes, and
near 2 minutes in metasilicate.

But I find hard to know when developing is over, because I see a light
gray layer over the bare copper areas, and I don't know if it is
finished or not. And  don't know if I can safely clean it with a soft
paper, or this way I can take it off!

Any help about this to get better next time??
Thank you!!