There are a good number of readability formulas in the market for technical documentation. See readable.
You must select the one that meets the requirements of your documentation standard. To select the best formula, you must weigh some of the primary factors of your audience type.
You must know the education level of your audience, their ability to understand English, how clear technical informal should be, and ease of usability of the devices.
Primary Factors of Your Audience
(1) Education Level: You must be clear whether you should describe the features directly or include examples in good numbers. You must write concisely or explain the things well. Whether you should cite use cases or it should be explanatory in nature.
(2) English Understandability: Whether English is the native language of the audience, or it is a second language. If your audience is global, you must use neutral language that appeals to all. Neutral language is free of humor, cliché, phrases, color, and stereotype.
(3) Technical Information: Whether your explanation should be of a high standard or simplified one. If your audience is commoners, technical information should be written like story beginning with an issue, solution, and happy experience.
(4) Ease of Device Usability: You must know how easy it is for your audience to access the software or apps that they use. What kind of computing devices they would use.