If memory serves, the CG quoted for the Northstar on the plan is for the dry aeroplane, tank empty, and is 50 mm ahead of the step, and they warn against using a bigger tank than (about) 250 ml to avoid getting tail heavy.
I used to place a pencil on the bench and balance the model on that.

That CG at 50 mm is quite safe, even sluggish after a while and it needs a push of down to hold inverted.
After a bit of flying experience you might want to move the dry CG aft a little, to 40 or even 30 mm ahead of the step