There's guesswork involved but you can identify individual patterns within squares and make educated guesses. It's hard though - I struggled and I created it!
Finally completed it...very tough but once i got the first sort of 5 boxes i was determined to complete them lol. congrats on having the hardest one i've done so far lol.
A little help please: now I have filled all 25 squares, but there must be a mistake and I can't find out where!!! There are only a very few possibilities to change the black points and I think I tried all - but it seems that I didn't find all...??? Am I right that the most of squares are regular / mirror-symmetric? One seems to be a spiral?