Don't calculate by hand unless you have nothing else to do. Do use the grammar checker readability tests in your word processing program. Expect different formulas to produce different results. Select a formula that is designed to test materials similar to yours for learners similar to yours. Expect different scores for different formulas.
Different formulas result in different scores.
Various government departments, military agencies, business and education organizations use readability formulas to measure the relative difficulty of deciphering the words in their documents. So far, no formula is specifically designed for health and medical materials. These formulas require a sample of at least 200 words for accuracy, and several samples for pieces larger than a brochure. Rudolf Flesch was not the first to measure readability - that honor goes to Talmudists who around 900AD recorded counts of the number of occurrences of words in their scrolls. Flesch is credited with the most widely used and well-validated modern formulas.