UPDATE:
I tried the following values and combinations on both a 100% cotton shirt, cotton blend shirt and a cotton fleece hoodie, all using the better PET FILM (without the pattern line design type one) and cured using my usual, tested curing settings (time and temp):
Light press:
1440 (20/30 ink level)
1440 (20/90)
1440 (70/90)
Heavy press:
1440 (20/30)
1440 (20/90)
1440 (70/90)
First off, all the combinations printed w/ both light and heavy presses on both shirt and hoodie came out better, more durable and stretchable compared to the inferior PET film with line pattern design.
Here are the output from best to worst (most durable / stretchable / solid color / solid 'opacity)
Heavy press 1440 (70/90)
For the best output (Heavy press 1440 (70/90) everything came out around 95% ideal for my standards. Perfect color, perfect thickness, durability that allows for heavy stretching and scratching
Heavy press 1440 (20/90) & 1440 (20/30)
The second output (20/90 and 20/30) actually have a nearly similar durability and stretchability with the first one, and is just very very slightly less durable because of how thin the ink is. What makes it worse is the color. Magenta and cyan came out as baby pink and powder blue
Light press 1440 (70/90)
Slightly similar finish with 2 BUT is again slightly less durable
Light press 1440 (20/90) & 1440 (20/30)
With a very inaccurate, light color and is less durable because of how thin the print is. Using this setting, one design printed on a cotton blend fabric got damaged / cracks after heavy stretching.
The light press settings came out obviously inferior to the heavy presses. BUT if compared to my older lightly pressed design using the cheaper PET film with line pattern design, it still significantly has better durability and stretchability, although it still doesn't pass my expected quality standards.
In conclusion:
Heavy pressing appears to improve durability and stretchability (as correctly suggested by @johnson4)
Having 1440 setting helps the ink print hold enough to avoid small holes and mini cracks to appear after heavy stretching
Fabric / material appear to have minimal to no effect when it comes to durability of the print.
Still unproven, but based on my experiment, using PET film with a better quality, and having avoided the inferior PET film with the line pattern design, resulted to better durability / stretchability of the print across the board when compared to my previous prints using the inferior PET film consumable.
Will update should something new come up when I print new designs soon.