To me 500 ug at once was fine, but it shouldn't have been more either. Honestly I can say that I didn't knew what full synthesia was before that experience. The constant buzzing vibration, hearing certain noises like bleeps or zooms when looking at certain objects and feeling the head as a spaceship. The heart pulse clearly connected with the brain center.
The whole 'double-sided' eye-sight, literally seeing crawling grey climbing plants on the white wall, and chequered patterns on the floor rollin'. Big and small fluctuated a lot. Everything went 'slow'. Though all with a bright awareness that it was the acid. CEV's just silver and a white spectrum, though some lines purple and yellow. Completely different than 250, which was way more colourful and less physical. And even 375 (flowerpower!) was still more comparable with 250 than 500.
It was also the only and first time in where I felt the 'unprotected' half of LSD's chemical character, it made sense how it could destabilise one's identity area in a degenerate manner.
I wonder what would come out when crossing this amount, but to me it felt as this was more or less the maximum potency of acid as a classic psychedelic, I suspect the 625-1000 range would act more as a dissociative psychedelic. I like 250 ug the most, I don't have to let go entirely so keeping focus on the music, insights and CEV's goes pleasantly smooth.