I'm not sure of any articles that suggest ppm but there are growers on this forum that can give you suggestions about that.

I would not use Urea on any bulbous plants as it promotes growth of bad bacterias that create bulb rot and it isn't readily available to the plants.

I would try and use a fertilizer that is sources nitrogen primarily from nitrates, Dyna-Gro 'Grow' is a good all about fertilizer which utilizes nitrates. This is what I use.

How about a book?

Veronica M. Read: Hippeastrum - the gardener's amaryllis

That book would be great, however I live in Lithuania (Europe) and shipping costs would be insane (most likely more than the book itself) and for that money I could buy a lot of hippeastrums :D
Also, we don't have a lot of the things that you have in the US, for example Dyna-Gro 'Grow' fertilizer...

Well, I live in Europe too, we're almost neighbours ;D, and didn't have to pay myself sick to get it. Internet and online-shopping makes it so easy...

Also, I'm not sure how much it's worth believing what writes a person, who has stated in an article about propagating hippeastrums by bulb cuttings, that you shouldn't blow air in a bag with pieces of the bulb in vermiculite if you have a cold, because it infects the cuttings. How on Earth a human infection could affect plants is beyond me!

This article is old, but has some mention of fertilizer strengths.

Here is a link that might be useful: Understanding and Producing Amaryllis