My question: Are there ways to point out errors of somebody's scientific work without sounding harsh or as if I am discrediting that person? A lot can be found about criticizing well and proper feedback, but that usually concerns a face-to-face conversation between two people who know each other. My situation concerns a scientific work of somebody unfamiliar.
The situation: I have written a scientific article that corrects and improves a computer model of another scientist, say of Johnson. To justify the relevance of my article I have to point out the errors of the former model. Currently I say this like "Johnson errs twice by proposing equation ...", "Johnson's model lacks ...", "Johnson employs a method unfit for the purpose of ...".
The issue: I thought my wording was clear and business-like, I also start by giving credits to Johnson for being the first with his model. Yet three proof readers found my criticism harsh, one of them even said my words are discrediting Johnson. Clearly I am currently not attaining the right goal, people now read it as harsh criticism instead of an explanation what is wrong with Johnson's model.
My (somewhat) fix: Instead of "Johnson makes two mistakes by proposing equation ..." I could say "Johnson proposes equation x. Equation x contains two mistakes.". That is, I tag his equations, and then shift focus from the person Johnson to his equations, thereby making it sound less personal. Likewise, instead of "Johnson's model" call it "the former model". From a writing standpoint, however, this sometimes reads artificial.
To make matters more complex: Johnson's model has flaws yet he presents very good results, sometimes unusual good results. I explain why this is unusual. I do not want to sound harsh here, yet I also do not want sound as if his results are plausible.
Personal sidenote: Rephrasing my wording sometimes feel like twisting myself into difficult postures. That I have to omit the word "Johnson" to sound fine. And even when Johnson did a poor job I should obfuscate this.