generally the guides do leave out the VR rings for whatever reason. not suggesting they aren't good, or even BIS for what they do, but maybe because you can't even have multiple?
either way, there's seems to be a lot of good information here and most will vary depending on the numbers you're already working with.
a quick guide for damage though, if you're seeing inconsistent numbers, just for arguments sake, 1000, 500, 250, 125, with varying amounts of each, that means your magic acc is lacking and you're getting a variety of resists.
if you're seeing more of the 500, 250, 125 and less of the 1000, it means you're lacking int and magic acc because not hitting the int check means you'll never get better than a 1/2 resist
if you're seeing all 1000's, that means you're at the int/mag acc check and you can add more mab and magic damage, possibly at the cost of acc or int.
now clearly 1000 is just an arbitrary number I used to demonstrate the resist values.
that actual value you would need to test with your gear, and unfortunately for you, kaustra isn't something you can spam to test. that being said, you can look at the magic damage page to at least get some kind of an idea.