I'm planting potatoes in grow bags for the first time this year. I'm doing the thing where you start by filling the bags only partially with soil, then add more soil as the plants grow, until the bags are full.

I've read that it's time to add more soil when the plants are 8" tall. The thing is though that the plants are of different heights. In many of the bags, one stalk is almost 8" tall, while others in the same bag are just emerging from the soil.

Do I add more soil when the TALLEST stalk reaches 8", or when the SHORTEST stalk is 8"?