Hello
Did you get 2 separate samples? The ata-xl and then the pk 5-4?
If so you can use the xl throughout the life of the plant but maybe don't start until the plants about 4-6 inches tall and has a few sets of true leaves.
The pk you want to start that right as the first pistils show up, start with low doses and then buildup each week.
But when you're in the seedling stage you want to do zero or very minimal nutrients.