"As most know, I almost died from a failed water fast due to my bp dropping too low. So my question is how low is too low?"
Wow, i did not know this and nor have i heard of this before. When did this happen? Might you perhaps paste a link to the post where you mentioned it? I don't have an answer, but do you recall how low it got the time it was a problem? Was this low blood pressure accompanied by any kind of activity that might be considered excessive in a fast or was the bp an issue in and of itself (as i believe you are saying)? Very good for fasters to keep this in mind as something to be aware of.
"I am having an extreme case of painful eyes. Now is this from the use of the computer or low blood pressure."
I believe in a fast our muscles are quite a bit more easily fatigued and this would apply to our eye muscles as well. But "extreme pain" is far beyond anything i have personally experienced.