Listen, IMO it doesn't really matter how bad you and your team play, how many times you die, or whatever as long as the damn mission gets completed along with all of the side objectives. and extracting a decent amount of samples with everyone extracting. Even if there are only 2 reinforcements left it doesn't matter because it is a completed mission and contributes to the war. Now on an individual level if you are worried about your stats I would say worry the most about kills, deaths, and accuracy.
Deaths and accuracy are the most important because you shouldn't die 8 times every mission. After all, that means you aren't killing enemies fast enough, you're playing too recklessly, or your teammates are being shit and don't know how to throw napalm barrages correctly. The fault is usually on you though. It also means you're eating away from the reinforcement pool of the squad. Accuracy is important because a missed shot is useless and does no damage. If you're new 60% is a good starting point to gun for, and after you get more experience crank it up a notch and shoot for a consistent 70-80%. Kills are only useful if they're near your teammates and take pressure off of them. It doesn't help your squad that you are killing enemies 500m away alone unless you're drawing all the enemies with you away from them. Soloing and having to kill a ton of enemies on 4 man 9+ missions risks death and loss of samples. This is especially true with bugs near the start of a mission due to stalkers. Don't go it completely alone if you're not 100% confident yet.