Text complexity as a research problem is equally relevant in linguistics and education since its solution provides an algorithm to match readers of certain categories to texts. Numerous studies have been conducted to identify quantitative and qualitative parameters that affect text complexity in ESOL. However, text complexity range within one proficiency level remains a research niche. The current study is aimed at identifying the range of text complexity fluctuations within one proficiency level and their appropriateness for readers. We conduct a multi-factor analysis and contrast 66 English texts for Basic State Examination (OGE) in readability, average sentence length, word length, number of verbs and nouns, cohesion and lexical diversity. The text features computed with the two online services, Text Inspector and Coh-Metrix, provide slightly different quantitative results though consistent in qualifying the range of text complexity fluctuations as high. The research findings refute the hypothesis of a linear nature of text complexity growth in the textbooks designed to increase students’ proficiency and confirm the lack of correlation between the revealed and claimed complexity of texts. The algorithm suggested by the authors can be useful for textbook writers and test developers selecting reading material for any proficiency level.