TTC after mirena?

Hey everyone I'm new to this.   I am married to a wonderful husband and we each have 2 kids-none together..  We finally decided that we would like one more child.  My youngest is almost 8 and I have had my mirena in since right after he was born.  (Technically this is my second mirena). Anyways,  I was wondering when would be a good time to really start trying? I haven't had a period since getting the mirena so I have no idea how to chart anything.  Any advice?