Should I be looking to get a frame with the same effective top tube as my road bike (58.6cm)? Or shorter or longer?

I should probably clarify that the frame I’m looking at (Dolan Pre Cursa) is either 57cm or 60.5cm, so my existing frame (which sems to fit fine) is almost exactly half way between the two choices. So I have to go either larger or smaller…

Go smaller. Long stems are fine on a track bike. I have a 54, and a 52 with a 120 stem. They both ride fine.

Reach should be the same as your road bike when getting started, but you may want a lower front as you progress (although my nice track bike is set up the same as my road bike).

Nice frames too, have a pre-cursa and a Seta.

57 with longer stem, the 60cm PC has a huge headtube.