I've written a script to solve this problem. It can be found on GitHub here. If you are not a developer, please use the website above!
It tells you what games are below average, and which ones are the easiest to boost (strictly mathematically). Improvements and pull requests are welcome. I know very few people will want to download something for this, but it's the best I can do without having a server. Here's a sample run of the code:
$ node index.js --api-key=XXX --vanity-url=seiyria
Resolving vanity url "seiyria"...
Getting games for steam64id: 76561197998412147...
Found games: 416 (not all of these will count)
Found 95 games that count for Avg. Completion Rate, sorting...
Your average percent: 29%
Games below the average (sorted by easiest to boost, numerically):
Adventures of Shuggy: 3/13 23.08%
Monaco: 2/13 15.38%
Worms Clan Wars: 3/14 21.43%
Tower of Guns: 4/17 23.53%
Frozen Synapse: 2/16 12.50%
Rochard: 3/18 16.67%
Dungeon Hearts: 2/17 11.76%
Lost Marbles: 2/17 11.76%
Cogs: 1/18 5.56%
Wizorb: 2/20 10.00%
Scribblenauts Unlimited: 4/25 16.00%
Puzzle Dimension: 3/25 12.00%
Legends of Aethereus: 1/23 4.35%
Shatter: 7/30 23.33%
Stealth Bastard Deluxe: 4/27 14.81%
Tiny Barbarian DX: 4/27 14.81%
Dwarfs!?: 5/29 17.24%
Orcs Must Die! 2: 3/27 11.11%
Crypt of the NecroDancer: 2/28 7.14%
Wanderlust: Rebirth: 3/30 10.00%
Foul Play: 3/30 10.00%
I am Bread: 8/35 22.86%
Trine: 4/33 12.12%
Pixel Piracy: 1/30 3.33%
Blocks That Matter: 1/32 3.13%
Half Minute Hero: Super Mega Neo Climax Ultimate Boy: 3/35 8.57%
Runespell: Overture: 1/35 2.86%
Tower Wars: 1/35 2.86%
Hotline Miami: 1/35 2.86%
Hammerwatch: 4/40 10.00%
Atom Zombie Smasher : 4/41 9.76%
Just Cause 2: 12/50 24.00%
Gauntlet™ : 5/43 11.63%
Hero Siege: 9/51 17.65%
Sleeping Dogs™: 16/59 27.12%
Super Meat Boy: 4/48 8.33%
KRUNCH: 4/51 7.84%
One Way Heroics: 11/59 18.64%
Darksiders II: 1/50 2.00%
Torchlight: 16/66 24.24%
Alien Swarm: 15/66 22.73%
Left 4 Dead: 15/73 20.55%
Worms Reloaded: 1/61 1.64%
Beat Hazard: 1/63 1.59%
Magicka: 25/88 28.41%
Rhythm Zone: 10/80 12.50%
Saints Row: The Third: 3/83 3.61%
The Binding of Isaac: 2/99 2.02%
Sanctum: 1/100 1.00%
Reus: 16/123 13.01%
HOARD: 9/129 6.98%
A Valley Without Wind 2: 1/210 0.48%
Sid Meier's Civilization V: 34/287 11.85%
Killing Floor: 21/285 7.37%
Games above (or at) the average (sorted by easiest to boost, numerically):
bit Dungeon II: 13/15 86.67%
Eversion : 11/14 78.57%
140: 3/6 50.00%
Cities: Skylines: 29/33 87.88%
Grotesque Tactics: Evil Heroes: 4/10 40.00%
Papo & Yo: 4/10 40.00%
Rogue Legacy: 21/28 75.00%
Nidhogg: 5/12 41.67%
Pid : 4/12 33.33%
Portal: 6/15 40.00%
Chime: 6/15 40.00%
DLC Quest: 13/22 59.09%
TowerFall Ascension: 20/29 68.97%
Vertical Drop Heroes HD: 19/30 63.33%
SteamWorld Dig: 12/24 50.00%
Bastion: 9/24 37.50%
Pillars of Eternity: 16/31 51.61%
BIT.TRIP Presents... Runner2: Future Legend of Rhythm Alien: 10/26 38.46%
Quantum Conundrum: 8/25 32.00%
Mark of the Ninja: 21/38 55.26%
Super Puzzle Platformer Deluxe: 27/44 61.36%
Shovel Knight: 27/45 60.00%
La-Mulana: 44/64 68.75%
Transistor: 10/33 30.30%
Thief: 13/37 35.14%
Portal 2: 26/51 50.98%
State of Decay: 20/46 43.48%
Gunpoint: 15/42 35.71%
Dishonored: 51/80 63.75%
Goat Simulator: 58/87 66.67%
Dungeon Defenders: 73/104 70.19%
Borderlands 2: 33/69 47.83%
Borderlands: 33/80 41.25%
Left 4 Dead 2: 22/70 31.43%
Torchlight II: 70/119 58.82%
Defense Grid: The Awakening: 35/87 40.23%
A Valley Without Wind: 32/98 32.65%
Trine 2: 30/97 30.93%
Team Fortress 2: 156/513 30.41%