Thanks for your comments!
About the tar method: I used a wooden wedge (oak) on the handle too. The tar and sand are supposed to act like a glue. It's done by smearing some pine tar onto the handle bit that goes into the head, heat it shortly over the fire, dip it into a bit of fine quarz sand and then shortly into the coal ashes of the fire and put the hammer head on. My teacher told me to file the head so it goes about two thirds through the head easily, the rest is then whacked in. The wedge gets the same tar and sand or ash treatment and is then hammered into the handle, cut or broken off and the the remainder of the wedge is used to hammer the bit that is in the handle a little deeper still. Then the handle is hammered a bit so it spreads over the wedge, locking it in even better. You can sort of see it a bit on the second picture.
I can't tell how it is holding in the long run because I made it only recently, but so far it's fine. All the hammers I have used in the classes before were factory made, so they were not fixed this way.
The head weighs 1580 g, by the way. I forgot to mention that earlier. I was aiming for 1,5 kg when I set out, but it's a bit hard to estimate where to cut, because I started with a longer piece of steel.
I don't know about the traditional Finnish pattern. I based the design on my teacher's hand-made hammer with some inspiration taken from Hofi's hammer design principles. I tried to keep the mass somewhat even on both sides of the handle and I was advised to keep the distance between the handle and the flat face short. The cross-pein is a little longer than I initially aimed for, but shorter than the one on my teacher's hammer. Don't know how traditional the design of his hammer is either. I guess that's how things evolve, copying from more experienced people with slight modifications.