pcb design - How to properly develop a PCB everytime without taking too much photoresist off

Its been a while since I made a good PCB and I can't remember what I did to make it perfect, but after reading various resources, I have determined the following as to why most of my design was removed in the photo development process:

Maybe my lighting unit is incorrect. I bought a black light from a party store that has a long black tube and internal circuitry. I remember once trying a black light in a halogen bulb with kind-of better results.

I then printed two identical copies of my circuit (I chose mirror mode in software) on transparencies in a 600DPI laser printer and tried the process with the toner side towards the light (maybe I should flip that around?)

I then made myself a lightbox for my circuit board by inserting my blacklight inside a "shadow box" from a decor store called Homesense. I'm not sure if this will be a problem. In the past I used regular glass with good results, but someone online mentioned plexi-glass is better for circuit boards but I'm not sure how much better if any.

I then placed my board about one inch away from the light to make sure the light shines on the board. Is this too close?

And finally, what kills me the most is exposure time and development time. I then proceeded to expose my board for about 45 minutes because people say longer times are better.

Since I had no measuring tools, I filled a bottom of a 5in by 9in plastic container with water about 2cm high, then I slowly poured developer for about a second. I placed my board in. For the first 5 seconds, nothing happened. then I slightly shaked the container to mix the contents and to my dismay, most of my design disappeared All I saw were my large ground tracks. Since it was a waste, I might as well continue. So I did and about 10 seconds later those tracks were removed.

So here's my question. For the developer stage, what's the minimum amount of positive developer and water I need for a board 15cm long and 7cm wide? Is there an equation I can use? I can't seem to find one on the internet except all the bottle instructs me to do is use 10 parts of water to one part of developer.

Also, what should I look for when the board is in the mixture to indicate progress or error? (I want to take it out the moment it is likely to be overdeveloped)?