Hmm it's a bit of a puzzler with coco! I'm using biobizz lightmix soil (for a change) and know the seedlings can last 20 days plus with nothing but water.
As you say, it's a trial and error. I have no idea if the biotabs all being activated at the same time too close to a seedling would burn .... I just don't know - never really tried this method (it's as close to organic as i've been).
I do know that at 7 days, a seedling in coco can look underfed if not tended well ... but for me, tending well would have mean A&B, Rhizotonic, and ph&ec checked. That's not the easy biotabs way