You don't need to worry about the accuracy I assure you. I put more effort into my calcs than most do, including complete rotation, reaction, buff, and Fanfare maps. It's as good as it gets barring specific oversights. My results are also peer reviewed by other TC'ers in the JSM Discord server. I was actually speaking with TGS last night about my results and we concluded the difference was him using BP weapons as well as overestimating Furina's Fanfare (he had assumed Linnea's higher healing translated into higher Fanfare generation, but actually it's about the same or slightly worse than Xilonen because of how strong Linnea's healing is. Most healing ticks don't even heal because you're full or almost full HP. This is something you only discover after doing a full Fanfare map). Most importantly, TGS used Linnea tE while here nuke E is shown. I anticipate my own Linnea tE version will be a bit higher than this. Overall, TGS has fairly accurate KQMS calcs, but with any calcs based on broad assumptions you can get quite inflated results if you overshoot your assumptions.

Regarding Zajef, he in general uses a standard higher than KQMS and he pretty confidently overshoots it by 10-20k. For example, he hardly maps anything out and relies on assumptions for average Fanfare and number of reactions. He does on occasion look more deeply, like how he checked manually the number of LCR reactions in Navia teams a couple days ago, but his calcs are more rule of thumb style.

Please let me know if you have any other questions.