I swear EA got the achievement names/descriptions the wrong way around...As such, I'll cover "Devin's Grade" and "Colt's Grade" in one!
This is relating to the Longshot: Homecoming story mode.
For Colt, getting an 80.0 grade is easy. His games are easy, and I won almost them all first time anyway without a single fail. I finished with a 95.0 grade (we need 80.0).
You do lose 4-5 grade points if you do “fail”, but that penalty is only incurred once a game. So if you incur your fail penalty, and you fail another 3 times, you won’t get penalised further (from my experience with Devin, anyway, correct me if this is wrong). As long as you don’t occur a fail penalty in more than 2 of your games, you’ll be ok.
Make sure you are no lower than 60.0 going into the final game as Colt (against Beeville Bearcats). I went in at 75.0, and got 20 for winning it perfectly, pushing me to 95.0 If you want to be extra safe, just quit out before the end of that game and you’ll just restart it.
For Devin, getting the 80.0 is a bit tighter (but still easier than Madden 18). The same fail conditions apply, and I only failed the “finale” section (incurring the game penalty) and did everything else perfect, and got 86.0 (we need 80.0). The games are quite a bit tougher too, so this leaves less room for error. I recommend that every game pre-finale you try and do as perfect as possible. If you get the fail penalty, just quit to Longshot menu and retry.
Make sure you are no lower than 65.0 going into the final game (against New England Patriots). I went in at 71.0, got my fail penalty of 5 points and ended with an 86.0 grade. You can still get an 80.0 grade entering at 60.0, but getting 20.0 for a perfect win (i.e. never failing this game) is quite hard. If you come in at 65.0 or above, it’s guaranteed as long as you never skip and you can just fail the finale as any times as you like with no stress. I can cateogrically confirm the max you'll be penalised from the Finale is 5.0 points.