I have had a hard time with the tone thing in the past as well. One thing that worked for me is trying to think about how my favorite educators in the past would try and reach out to me to "lift up" my level of skill instead of "put down" the work that I had done. I know that you are correcting the OP because you'd like to help, but your help is more effective if it is seen as a helping hand and not a disapproving glance.

A basic formulae could be a) acknowledge the positive attributes of the attempt (it is great to see you trying to optimize, and you are right to keep detailed numbers about what was going on, your funnel analysis was also spot-on.) b) Point out that he could do better. This emphasizes the growth potential instead of the shortcoming (one has an implicit expectation that the person can and will do better, the other is frequently taken as a rejection of the person.) An example might be: "you ran the test, but your test would be more helpful if you did it in a more controlled way..." c[optional]) suggest additional resources so the person can follow-up at their own pace, in the manner they prefer. Ex: (see this great post about having a control when testing your site...)

Now, if someone has ignored your previous feedback and it is your obligation to help them, things get a bit more nuanced.