tested clocks (basically all combinations of 650/900 to 840/1100 with steps of 20):
650/900, 650/920, 650/940, 650/960, 650/980, 650/1000, 650/1020, 650/1040, 650/1060, 650/1080, 650/1100, 670/900, 670/920, 670/940, 670/960, 670/980, 670/1000, 670/1020, 670/1040, 670/1060, 670/1080, 670/1100, 690/900, 690/920, 690/940, 690/900, 690/920, 690/940, 690/960, 690/980, 690/1000, 690/1020, 690/1040, 690/1060, 690/1080, 690/1100, 710/900, 710/920, 710/940, 710/960, 710/980, 710/1000, 710/1020, 710/1040, 710/1060, 710/1080, 710/1100, 730/900, 730/920, 730/940, 730/960, 730/980, 730/1000, 730/1020, 730/1040, 730/1060, 730/1080, 730/1100, 750/900, 750/920, 750/940, 750/960, 750/980, 750/1000, 750/1020, 750/1040, 750/1060, 750/1080, 750/1100, 770/900, 770/920, 770/940, 770/960, 770/980, 770/1000, 770/1020, 770/1040, 770/1060, 770/1080, 770/1100, 790/900, 790/920, 790/940, 790/960, 790/980, 790/1000, 790/1020, 790/1040, 790/1060, 790/1080, 790/1100, 800/900, 800/920, 800/940, 800/960, 800/980, 800/1000, 800/1020, 800/1040, 800/1060, 800/1080, 800/1100, 820/900, 820/920, 820/940, 820/960, 820/980, 820/1000, 820/1020, 820/1040, 820/1060, 820/1080, 820/1100, 840/900, 840/920, 840/940, 840/960, 840/980, 840/1000, 840/1020, 840/1040, 840/1060, 840/1080, 840/1100