Plan to arrive at Lunchtime. Very, very rare to have an Alternate requirement sprung on you then.
Think about the other way, say there already IS a requirement on the TAF for your arrival, the TTF can change for the better too. You can depart hoping you get that improvement. But if you decision point is outside the three hour validity its no good to you. If your very close (67nm) alternate is miraculously improves however...
I have no idea if this is going to be the case, but based on the geometry it would be similar to the SIN - PER scenario where YPLM was our only option and we would sweat on getting a clear TTF just prior to the DP.