To help determine what grade level tests to administer depends to a great extent on what information you already have on a student. For instance if you have a 4th grade student, and you know they are functioning below the 4th grade level, perhaps start testing them at the third grade level and go up or down in grades and/or skill levels depending upon the results you get from their testing. To compare test scores with students in the various grades levels you can refer to the Progress Monitor Scoring Guidelines”. This should give you a ballpark idea where his skills vs. grade level lies. You will have to play around with various tests but eventually you will be able to determine where his skill and grade level aptitude are.
If they are functioning at the 50th percentile in a lower grade try them in the next grade up to see how they score. For reading, the skill sets stair-step up in difficulty beginning with the fundamentals of reading: Letter Names, Phoneme Segmenting, Letter Sounds, and progressing up to the more difficult skills of Word Reading Fluency, Passage Reading Fluency, and the hardest of all, Multiple Choice Reading Comprehension. They might possibly do better in higher grades of basic reading skills but need to drop down to lower grades on the more difficult skill sets. You are not only trying to determine where their knowledge lies but also their ability to read and understand words and sentences.
For a student not functioning at their grade level, you can assess them at different grades but the goal will always be to get them back on their expected grade level as soon as possible and at the 50th percentile.