This video shows an adjusted Pentacon Six/Praktisix shutter - as accurate as such a totally mechanically controlled shutter can be. Before adjusting, the whole mechanism had to be cleaned and relubricated with a modern, synthetic watch-oil. This is because the original lubricants are at least 25 years old! You don't dare to start the engine of an old vintage car after standing in a garage for 25 years, do you. I think the least would be to put some new oil in. ;-)
And to answer the question "how accurate is accurate?" here is the table of shutter speeds from the original Praktisix service manual. As you can see, it is not really neccessary that each single speed is perfect - more important is that the "distances" between the speeds are alright.
speed...ms...range of tolerance
1..........1000....750 - 1250
1/2.......500.....375 - 625
1/4.......250.....187 - 312
1/8.......125.......94 - 156
1/15......62,5.....47 - 78
flash......45........38 - 52
1/30......31,3......23 - 39
1/60......15,6......11,7 - 19,5
1/125.....7,8.......5,8 - 9,8
1/250.....3,9.......2,6 - 5,3
1/500......2 .......1,3 - 2,6
1/1000....1........0,65 - 1,3
