When to refil tank?

I am using the Smok Alien with the baby beast tank that came with it and X4 coils. I was wondering how far is ok the let the <a href="http://www.vapininthecape.com/eliquid_c_7.html”>juice go down before refilling it? I've generally been letting one it go half way down but if I want to change flavours then I end up throwing some away, and when this happens multiple times it ends up with quite a bit being wasted. Thanks <img src="https://www.planetofthe<a href="http://www.vapininthecape.com/eliquid_c_7.html”>vapes.co.uk/forums/new-users-corner/new-user-questions-and-discussion/data/smilies/1.gif” class=”mceSmilie” alt=”:)” title=”1 :)” />