That's why I have been taught to trust in the telltale method. Sail makers don't just stick em any old where. Each tell tale is positioned to guide you to achieve optimum sail shape and control.
Take the very top telltale, middle if you have three in a row what's it telling you?........ If the outside is flying horizontally and the inside isn't you've either sailed off the wind (pinched) or you haven't enough mainsheet tension.
Now you are beating nicely the mainsheet is pulled on as hard as is reasonably practical for the conditions and the top telltale still wont fly horizontally. If you pull more downhaul on from your marked max power setting (we've all marked our downhaul position for max power haven't we). Then the inside top telltale WILL fly horizontally.
And so on, each telltale serves a purpose.