Unless the fern is distressed, 1/2 strength feeding is usually ok. Ferns don't need much food, but light feeding in Spring helps them.

If the fern seems weak, 1/4 strength (or less) feeding is the most you'd want to give it until it looks like it's growing.

Overfeeding is much worse than underfeeding ferns.

I don't know those species of fern, but when it comes to general informtion I'm not surprised it can be so variable. Take Acrostichum speciosum as a example, if you gave it a big dose of fertiliser it would laugh it off. It's a mangrove fern that grows in brackish/saline water habitats. It's evolved in a high nutrient environment so copes reasonably well with chemical imputs. Other end of the scale are the rainforest epiphytes growing on tree trunks where all they get is decomposing vegetation that's well watered down by the frequent rain. They'd curl up their toes in all but the very weakest fertiliser. Generally with ferns, all you can say is to be very sparing with the fertiliser but very generous with water. Of course drainage capacity of their growing media is another story. But as a generalisation the faster draining the better whilst always maintaining dampness.