When should I increase light and to how much and at what stage for my indoor lettuce?

I am growing 9 1.5g containers of ‘Marcel of Four Seasons’ lettuce. They are in a 2.3 x 2.3 grow tent using an LED. They are only seedlings at the moment. Currently they are getting a DLI of 10-11.5 with 16 hours of light (the middle containers getting more.) I know for now this is ample light. I’ve read that ideally 12 DLI is the minimum and have read that mature lettuce can use up to 15-17 DLI when they’re mature.

My questions- What size should the plants be before increase the light levels? Also, what should the maximum DLI be? Thanks.