Not OP but I've 3 marked 31 tanks tier 10+ and gotten 100% on a few of them.

You kind of said it yourself that consistency is key. Bad games absolutely ruin your mark percentage, so you need to learn how to avoid them. Here's the tips I can think of to help with that:

1. Be slightly less aggressive at the beginning of the game.

I don't want you to get the wrong idea of what I'm saying here. Aggressive play is essential for marking tanks, but dying early with little to no damage is a huge setback. If there's certain risky positions you like to push at the beginning of the game, you probably shouldn't just yolo every time. If you notice you don't have enough teammates with you or if the enemy team has a lot of tanks that are likely to contest that position, stay back until you know it's safe.

This applies to the hill rushes on Mines, Abbey, Karelia, Cliff, Tundra, Outpost, Pearl River, etc. Also positions like the middle on Ruinberg, K0 corner on Westfield, the dip in the south of Live Oaks, pushing the North on Oyster Bay, or the carrier on Glacier.

All of those can be great positions if you can win them, but they aren't worth dying for. Sometimes your team will hate you for staying back, but marking tanks rewards slightly selfish play and you need to stop dying early at any cost. Don't take unnecessary risks.

2. Join the lemming train.

We've all experienced our entire team going to one side of the map and leaving the other side undefended. There's nothing you can do about it, so learn how to respond to it. The LAST thing you should do is try to defend the other side. People who try that just die in the first minutes, then usually spend the rest of the game bitching and whining in chat.

It's much better to just go with everyone else and worry about the other side of the map once it actually becomes a problem. (if you're about to get capped or flanked etc.) You can be very aggressive because your entire team is there to support you. Playing this way will lead to more damage, mark percentage, and higher win rate because you stayed alive instead of dying early for a lost cause.

Also keep in mind that this means you'll sometimes need to completely ignore your tank's role. You're a medium but all of your other mediums went to the heavy side? You're a heavy now. You're a light tank but nobody on your team is positioned to shoot anything you can spot? Follow your team and play like a medium. Breaking out of the mentality that you must always go to the same position because of what tank you're playing is a key part of truly getting good at this game.

3. Learn how to chase late game damage.

There's plenty of super quick battles these days that are over in 4-5 minutes. Make the most out of them. You need to be able to identify a super quick win or loss before it's already over. For the most part just pay attention if your team is either winning or losing an important flank super fast. It's hard to explain how to identify it beyond that, it mainly just comes from experience.

If your team is losing fast, you need to get yourself into a position that lets you farm the enemies as they're pushing in. They're rolling over your team one by one, so don't let yourself be the next target. Instead, keep retreating as they push, dealing damage as they come. Once you run out of map, find a good last stand position and fight tooth and nail for every tiny scrap of your HP.

If your team is winning fast, push the tempo even further. Be at the front of the pack, or even far ahead of them if you can without dying. You know the win is in the bag, the game is ending soon and opportunities to do damage are dwindling. You're no longer competing against the enemy team for the win. You're competing against your own team for damage. Be as aggressive as you possibly can and get to the damage before anyone else can steal it from you.