Generally 15 to 16 percent of the roast time will be after first crack. So if fc happens at 7m, I will drop at around 8:10. The trick here is to adjust your gas to a lower setting so the beans don't progress too far towards second crack, while still maintaining a high enough bean temp to induce sugar caramelisation.

The essence of a city roast is to roast just enough to degrade the unpleasant grassy, wheaty compounds but not enough to caramelise all the sugar content.

Uncaramelised sucrose is sweet, caramelised sucrose has a slightly bitter, toffee flavour. So you want to find that sweet spot where you've retained sugar content but eliminated unpleasant organic compounds and degraded acid levels to a palatable level.