How to correctly calculate the width of text in pixels?

[colesnicov]

Advise on how to calculate the length of a string in pixels? I use this

uint16_t getStringWidth(window* _window, char* buf){
  uint16_t text_width = 0;
  _fontc f;
  
  for (uint8_t i = 0; i < strlen(buf); i++){
    if (0 > font_attr(_window, buf[i], &f)) {
      ESP_LOGW("Text", "Char '%c' not found in the font specs!", buf[i]);
      return 0;
    } else {
      text_width += f.skip;
    }
  }
  return text_width;
}

but sometimes it gives poor results.