I use full strength Blue MG.
Its not the mix it is how much.
Tricky on small seedlings.
I use the spout they have to make mixed drinks to measuer by way of counting.
I feed then water.
1/2 is fine too but they do need to eat like the rest of us when they are babies.
Just not as much.
In the little octagon cells it is just a wee bit, maybe a teaspoons worth or less.
Be prepared for them to grow like crazy.
Worth