You only ever need 3/5 Shadow Focus unless you're end-game raiding as a Shadow Priest. 3/5 will cover the 6% hit cap in any PvP build and while leveling it will make it so your spells hit on equal level mobs (yellow mobs) almost always.
Honestly...I have both 5/5 Blackout and 3/3 Shadow Reach in my leveling build, and I'll never look back. This is simply because Kronos is a PvP server and you're going to run into some world PvP while leveling. If you think you can just avoid that...well then you haven't gotten to STV yet lol. I also have 4/5 Spirit Tap because 80% chance seems to proc plenty often. I even leveled with 3/5 Tap for about 20 levels and noticed little downtime, as 60% chance is still good.
While solo questing...I open with Mind Blast (free cast cause no aggro yet) then immediately start back pedaling. While back pedaling I apply my instants...SWP, VE if I need it (I don't bother if my health is fine), then get 2 MF's off and wand till dead. 10% of the time I get Blackout procs mixed throughout...which allows me another full MF. I very rarely need to bubble or even fear the mob(s) off me, so that's mana saved there. Rank 3 MF and on starts to really out damage your wands unless you get really lucky on the AH with some of the blue BoE wands. It was at this point that I started to wand less and less and only if I felt I needed to save mana.
The game is 12 years old. I min/maxed during retail. This time around I favored survival and utility and not only is it tons more fun...but I don't feel like I ever die. I have no issues with mana so long as I'm not over pulling.