I have seen at least one post, with video, demonstrating that one does not need a 70% relationship to get the guns-blazing ending - it just requires the correct responses in the scene where one is sitting with Johnny at his "grave" (search YouTube for a post by xLetalis about "How to get secret ending").
In terms of the "f*cked it all up" answer by his grave, I think it is supposed to fall into the category of "tough love" ... I guess.
Another issue that I noted is that those guides that claim one has to give "correct" answers at multiple different plot points to increase your Johnny number seem to be incorrect. For instance, when leaving Clouds after looking for Evelyn, ones "Johnny number" will jump from 1% to 10% as soon as the elevator hits the ground floor - this happens before one starts talking to Johnny. Same thing with leaving the church after meeting Alt with the Voodoo Boys - even if one immediately takes the blocker pills, the Johnny number jumps to 15%. Got a similar 5% boost after talking to Hellman in the hotel, despite doing nothing but denigrating Johnny and telling him that I don't trust him. There was another 5% boost that happened somewhere else, though I cannot remember exactly where - I think it was shortly after the point where I rescued Goro (despite Johnny wanting me to run). Again, that 5% boost seems to be unavoidable.
I found all this because I was trying to play a hunch on a way to perhaps get one more ending beyond the known ones. I noticed in my first play through, when I was talking to Alt in Mikoshi that she said she could not save me because of "too many aggressive meds and too much time". It got me wondering what would happen if I kept my Johnny number and Time number as low as possible and never took Misty's pills. This, of course, completely cuts off all the Johnny, Rogue, Kerry quest lines, so only the Arasaka and Panam endings are possible. This is where I discovered that it seems to be impossible to keep the johnny number below 25% if one goes to find Alt and Hellman. I tried the Arasaka ending with a Johnny number of 25% and a time number of 40% but still got the "you're dead" ending (actually, even worse, since I did not run the Judy and Panam quests all the way, both were destitute and wallowing in misery in Night City).
If I can bring myself to try another run through, I was going to try running the Panam quests with my 25%/40% numbers and see what happens if I choose the Panam ending. Another approach is to not run the Judy/Voodo Boys/Alt quests at all and run only the Hellman line (which should cap my Johnny Number at 10% I think) and see what happens with the Arasaka ending. All this involves never taking Misty's pills, so the Johnny/Rogue/Kerry quest lines are still closed off.