I spent around 5 hours spread in 4-5 days to make this work, and i hope this info might help someone who got cought desperately as i did More on that later. Also check the bottom of the post for some PlotID bool values to use with Gibbed save editor for those who are desperate.
Here are the things you have to do to make her live in the end of Horizon Sanctuary mission:

To save Miranda, you MUST read the dossier on Kai Leng and meet her via a holochannel from the Specter terminal after the attack on the citadel. During the mission, she may also die if you do not persuade her father to back down. If that fails, you have to shoot him. If Miranda shoots her father, then she be fatally shot in return and die. What is the Mass Effect 3 Value for Miranda to edit in Save Edtior. How do I keep my fish from dying? Do I get more points with an imported character in Mass Effect 3? Saved Maelon's Research Data but Eve dies from the stress & fatigue of the testing.

• Make her fully loyal in ME2. edit It is reported that you do not have to make her fully loyal, but you must complete her loyality mission.
• Make her survive Suicidal Mission (obviously).
• If you romanced her at any point, do not break up with her. edit It is reported that you can break up with her in ME2, just not in ME3. It is also rumored that you can break up with her in the hotel room (meeting 3) but you cant do that during first meeting - this will kill her definitely.
• Meet her at the Docs in the Citadel BEFORE Cerebrus attack on the Citadel. Talk to her nicely and ask how you might help.
• Read Kai Leng dossier on your (or Shadow Brokers, not sure wich) terminal after you encounter him before talking to MIranda again. edit reported as unnecessary.
• Talk to Miranda through Spectre terminal on Citadel, and warn her about Kai Leng (make sure she responds with 'I will make myself ready' or something)
• Meet her on the Citadel later, and when asked for Alliance resources for some operation, give them to her.
• At the end of the Horizon, use Paragon/Renegade option while talking to her father.

This should keep her alive. If you miss any of the above mentioned, she dies. I did miss the first meeting at the Docs, and was stuck with her dead. So i did some research for my situation and while helping some people. Here are some Plot IDs you might find helping.
1. First meeting in the Citadel with positive outcome (ask her about her situation and promise to help).
20182 True
20183 True

Mass Effect 3 How To Keep Miranda Alive

20741 True

Mass Effect 3 Miranda Mod

22698 True
2. Second meeting via Specters terminal - we do not have the data yet, so if you do, please share.
3. Third meeting in person on the Citadel - give her the resources she asks

Mass Effect 3 How To Keep Miranda Alive As A

21872 True
21873 False

Mass Effect 3 Miranda Death

4. Miranda BreakUp: finally,thanks to Jereee, we have PlotIDs for breaking up with her in ME3.
If you did break up and you have changed your mind and want to keep Miranda alive, set
21751 False
Hope this helps Mad props to @gibbed for his tool and @rsanchezsaez for a working hack i used to export and compare plot bool variables, BossBast1 and Jereee for providing me saves at turning points.

All things fair 1995 trailer. Edited by vekkth, 27 March 2012 - 12:48 PM.

I've been playing through ME3, and I just completed Priority: Horizon. Unfortunately Miranda died, so I went back a few saves and have been playing it again in desperate attempts to try and save her. I spoke with Anderson about Kai Leng and read the dossier, and mentioned Kai Leng to Miranda at the Spectre Terminal holographic meeting, and I told her I'd share the Alliance assets with her when I met with her at the apartment. I've read that you apparently have to manually grant her access to the Alliance assets via the Spectre Terminal, however when I go there, the option isn't there? Is this a glitch in my game, or am I doing something wrong? Please help as I don't want to have played through again just to have Miranda die twice. ;~;