I go simply by trichome color (thru a 40x loupe) and by the look of the plant overall (ie: whether new pistil growth was occurring, if a large amt of the calyxes are swollen, if the pistil hairs were receding). My 2 strains were listed at 10-11weeks and 8-9 weeks by the breeder but actually traded flowering times (no the seeds did not get mixed up). The supposed shorter flowering time plant took 10 1/2 weeks (with one still going -->78 days today) and the longer flowering plant was undoubtebly ready at 9 weeks on the dot (this was under CFL too).
My trichs were atleast 70/30 cloudy/amber for all the plants I cut down and their calyxes were all swollen tremendously. The new pistil growth (appearance of white hairs) had also slowed noticeably for all the plants harvested.
I don't use a watering schedule either, I water when the soil feels dry enough and sometimes its every 3rd day and some of the plants (even in the same strain, cut from the same mother) took as much as 8-9 days depending on if it was producing buds or growing or establishing roots etc.
Lurking variables such as subtle phenotypic differences within the same strain*, undiagnosed or unrecognized stress in plants, medium of growth, nutrient levels, light source (quality and quantity), grower knowledge and experience, TLC (again quality and quanitity), grow room conditions, etc (the list goes on and on) can influence the flowering cycle of any plant.
*Plant genes recombine and can mutate in a similar fashion to human genes (or any other organism), just looking at the vast array of phenotypes in the human popultion can give you an idea of the genetic variability within a species and why the guidelines for flowering periods maintain plasticity.
Nutshell: go by what the plant tells you, b/c every plant/grow operation is different.
