Dunno, our final reserve fuel is 30 min, not 45 and alternate fuel can be considerably less than 15 min depending on distance to alternate.
Whenever it seems likely that we will dip into final reserve its a PAN situation and when we're sure we will use it, it is MAYDAY which gives us emergency authority. That does not mean we have to use it already, just that our fuel prediction says it is pretty sure we will, so remaining fuel can be considerably more than final reserve.
Of course we should tell ATC as soon as possible if there is any fuel problem, how to get to your alternate is pretty simple though, declare MAYDAY and intentions and simply fly directly there, we are still capable of navigating ourselves.