i find it's more about the health of the plant than the exact location i take a clone from. Age is part of it too.. if too young it'll be less successful. if all other relevant factors are fine, then i would expect that clone to root even if it was the terminal bud. if this is your only cloning opportunity on plant, it makes me suspect it's quite young. i would still take clones later, even if from first week of flower.
You can top whenever you want. just need enough remaining vege growth afterward to make it worthwhile. after flip you still have 3-4 weeks of strong vege growth for most strains. i'd guess somewhere around flower day 14-18 might be approaching break-even? really depends on each plant's timing a bit... Some peole do a second defoliation around that time too. depends on environment and lights since that impacts growth rates as does a plethora of other factors, so observe and react to the time frames you see with experience.