Since I stopped using pine shavings I've not had any freezing in my DL, even when it was subzero for many days in a row. When I was still using the shavings my DL would freeze in areas where the shavings were clumped.
I usually know to add more leaves when I see them getting thin around the edges of the coop and I'm starting to see soil peeking through here and there. That's where the chickens like to dig the most, so sometimes that's just where chickens have dug, but there's a certain depth of leaves I like to maintain, especially in the winter months to keep my bottom layers insulated and warm enough to continue composting. That's one indication I use for adding more leaves.
Another is a certain moisture level under the roosts, a darkening of the leaves there that seems to indicate too much moisture on the surface of the bedding. My most moist parts of the coop are under the roosts and around the edges of the coop on the long sides, so when those leaves start looking dark and compressed looking, I just add more depth and dry leaves there. I used to have to get down and feel of the bedding to determine this but now I tend to be able to eyeball it and see how it moves when I flip it~ if it's heavy or light~on the fork.
The middle of my coop tends to stay the most dry and the leaves there usually stay light and fluffy, but they start to crumble into smaller pieces, thus compacting down and are harder to move with my pitchfork due to the smaller particles. When all those pieces get so small I can't fork them easily onto the more moist places in the coop, I know it's time for new leaves.
When I empty my waterer, I empty it right there in the middle where it's the most dusty. It helps that area to compost more and it doesn't last at all, as the dust there soaks it up and wicks it right out of the leaf cover. I never worry about throwing water down in this coop, no matter how cold it is outside. The water is much needed in the middle and I can go up later and feel that spot and won't be able to feel wet at all. The dry leaves have been shifted over it by the chickens and the moisture is trapped into the soil and fine particles underneath the leaves.
I hope that helps. If I can I'll try to make another shorter vid about when to add and show what the leaves look like when it's time to do that, how they respond to the fork, etc.