[thiago.eec]

I have a Kindle Paperwhite 7th Gen.

This works 100% of time here:

If the source of your sideloaded book is an EPUB or AZW3, you can check the opf file for the <guide> section and set the bold entry:

<guide>
    <reference href="text/part0002.html#unrHhdowLssGAxfyCOhMBbB" title="start" type="text"/>
    <reference href="titlepage.xhtml" title="Cover" type="cover"/>
    <reference href="text/part0034.html#uAaqBmMhAvqGpEMFJtLckR9" title="Table of Content" type="toc"/>
</guide>

It doesn't matter the title, the file name, the ID (#someID) or anything like that. What matter is type="text". The file referenced with this type is going to be the first page displayed when you first open the book on your kindle device.

If you have an EPUB3 file, it usually has the same <guide> section in the opf, but in case it doesn't, you must look for the TOC file (usually toc.xhtml; to be sure: it is marked on the opf with properties="nav"). There, look for this:

<nav hidden="" id="landmarks" epub:type="landmarks">
    <h1 class="calibre4" id="calibre_pb_1">Guia</h1>
    <ol class="calibre5">
      <li class="calibre6">
        <a class="calibre7" href="cover_page.xhtml" epub:type="cover">Capa</a>
      </li>
      <li class="calibre6">
        <a class="calibre7" href="part0000.xhtml#aid-1" epub:type="titlepage">Folha de rosto</a>
      </li>
      <li class="calibre6">
        <a class="calibre7" href="part0088.xhtml#uDGp5c4xU8pnuYlEtOpUzGE" epub:type="copyright-page">Página de créditos</a>
      </li>
      <li class="calibre6">
        <a class="calibre7" href="part0001.xhtml#toc" epub:type="toc">Sumário</a>
      </li>
      <li class="calibre6">
        <a class="calibre7" href="part0003.xhtml#udUOo9WWW6Y5YZFfflORBe3" epub:type="bodymatter">Início</a>
      </li>
    </ol>
  </nav>

Here, you need to set the RED entry. The property used is epub:type="bodymatter". Again: it doesn't matter the text between the tags, the file name, the ID (#someID) or anything like that. Just use epub:type="bodymatter" to reference the file you want to be first viewed.