I don’t really worry about this because I don’t bring my plants back inside when hardening them. I start seedlings inside under a light. Once they are big enough, I move them outside to a semi sheltered area, usually under the edge of the roof and out of the sun during the hottest parts of the day. When they have a few sets of leaves or the roots start showing at the bottom of the pot, I put them in the ground. After they go into the ground, they are on their own to live or die.
I live in a very mild climate and, with exception to harsh midday sun, I don’t have to harden the seedlings much. Aphids are a serious problem and I don’t want them in my aerogardens inside.