Nettle 'soup' will be high in nitrogen, so use on leafy crops like lettuce and brassicas. Comfrey will be better as a general feed, especially for fruiting plants like tomatoes as it has a higher potash and phosphate content than liquid feed made only from nettles. I use comfrey feed on everything though, with great results. An easy-peasy rule of thumb for dilution is to water it down until it is the colour of weak tea. I make comfrey liquid in a large covered bucket. I simply ram it full of leaves which I crush as I put them in, then fill to the top with water. That makes a strong concentrate after a few weeks but I keep it at the bottom of the garden while rotting (it stinks to high heaven!) and then transfer to 2 litre plastic 'pop' bottles until it is needed. I've found by experience that it still works just as well after 3 years and probably has an unlimited 'shelf life' when in sealed bottles.
