Though it's not a hard-and-fast rule, I use the California ELD Standards to get an idea of what my students should be able to do. According to Krashen's theory of i + 1, with teaching and scaffolding students should be able to achieve one level above what comes naturally to them. That is where I design my instruction. If I have a student named Cesar who should be at the "expanding" level of the standards, I will scaffold his content up to the "bridging" level.
You can also look at student data in a program like Synergy or similar attendance program, do an informal survey or have conversations with your students, or do diagnostic assessments at the beginning of the year to get an idea of what each student is already capable of. Once you collect more writing samples from students throughout the year, you will get an even better picture of where they need adaptations and modifications.