Start low, reach your goal, and move up to the next one. Your goal should be set around 75 words per minute if you know your reading is lower than 7th grade level. Use timer for one minute and see how many words you read. If 75 or more raise your number to 100. If still more, raise the grade level you choose, then try again with 75 words in one minute and go up. You should always be able to grow, if you can’t, your level or your fluency number is to high. Lower it. Never change the amount of time you have, only the amount of words and the grade level.
This site has many links that take to free youth books.