It's guessable. The symmetry (diagonal) is broken once. That's how you can know that R25C10&11 have to be black and C25R10&11 have to be white (plus all 2x4 white squares around that. But this doesn't help very far. After you logically figure out (that's possible) that in R and C 6 the 3 has to be in the last 5x5square, you "guess" a big zig-zag line from R25C5 to R5C25. After that it should be easy.