How to tweek
I worked in a photo lab as a teen and I still diddle daddle in photography and love the Snapseed app. Noticed a preference for inverted (negative) filters when tweeking HPTs.
I prefer using high contrast filters and here's how I do it.
I use the Snapseed app and use the NOIR filter, select S04 at the bottom and put the wash and the grain at 0, keeping the filter at 100. Decrease the brightness until you get a good enough contrast to distinguish a line or not!
I'm happy to help anyone tweek theirs if you have trouble!