<p>Answers:</p>

<ol>
<li><p>When the answer is good or at least it is not bad (has not downvotes), then, yes. <a href="https://stats.stackexchange.com/questions/64538/how-do-i-find-values-not-given-in-interpolate-in-statistical-tables/64539#64539">Yours</a> seems pretty good. If the OP is sure the answer solves the question, then, yes again. </p></li>
<li><p>Yes. The Stack Exchange sites <a href="https://stats.stackexchange.com/help/self-answer">encourage this behavior</a>. There are <a href="http://blog.stackoverflow.com/2009/01/accept-your-own-answers/">rules</a> which avoid gaming the system.</p></li>
<li><p>Yes. When there is a better answer than the OP's. If another better answer appears after a self-answer is accepted, <a href="https://meta.stackexchange.com/questions/93969/is-changing-the-accepted-answer-frowned-upon">one can always undo the accept</a>.</p>

<p>If one accepts an answer when he/she shouldn't I guess votes will guide properly future readers towards good answers. <a href="https://stats.stackexchange.com/questions/20836/algorithms-for-automatic-model-selection/22633#22633">Here</a> is one example.</p>

<p>On Meta, I'd say if it is a discussion post and there is only the the self-answer there, then, it should have at least a minimum score, let's say, 3.</p></li>
</ol>
