I bump at first sign as well. For a couple of my girls, this means at pink belly (my Sugar is especially noticeable). For the others, I have yet to notice pink belly and their first sign is when they start to go opaque.

Also, I bump up humidity for everyone during this time with my humidifier on a humidistat, rather than worry about a single snake's enclosure. I go from my normal 55-60% RH up to 65-70% RH in the room. I have yet to have a shed not come off complete.