When you want to express an idea or opinion based on incomplete evidence or without a certain knowledge, you often use the word "surmisal." However, there are other words that can be used as synonyms to help you better articulate your thoughts. For instance, "conjecture" is a good substitute that implies guesswork or supposition. Another option is "speculation," which suggests a theory without evidence to back it up. "Assumption" is yet another option, signifying conjecture based on limited information. Lastly, you might consider using "presumption," which implies that you're making a logical conclusion based on a set of assumptions or evidence.