Hasselblad Prism Magnifier

You will find that the eyepiece magnifier helps a lot. I use the same one on my 45 degree prism. It mounts very easily either under the eye piece via a hinged ring or via the cold shoe flash mount. The magnifier is on a hinge regardless of mounting method used. This will allow you to obtain critical focus and then flip the magnifier out of the way for composition. I find the prism finders almost worthless without them.

There is a number of dioptr lenses available to make use of the finder possible without glasses. Maybe that is an option to consider. The prism magnifier cannot be used with a PME 90.

You can indeed use the magnifier (cat.no. 42459) with the PM90.

Using one of these, you do indeed only see the center of the image.
So it's for focussing purposes only: you need to swing it in front of the eyepiece to focus, swing it back again to see the entire picture, back and forth, all the time.
It does indeed help in focusing, but i rather would avoid using one if possible in anyway.

The higher viewfinder magnification of the 45 degree prisms does indeed help. But perhaps not enough.
Apart from selecting and installing the correct diopter, like Paul suggests, using a split image rangefinder screen is a great help too. But it too has its drawbacks: one of the split image wedge halves will black out with lenses slower than f/4; and you need to find clear lines in your subject that run in a convenient direction.
And the thing about them that bothered me most, before my eyesight went downhill in a hurry, was that the split image wedge is right in the middle of the screen. A big distraction when wanting to see the image as a whole. But now that my eyesight has gone, i just love these split image thingies! ;-)

thanks a lot! which split/grid focusing screen would you recommand? : )

An Acute Matte (D) screen. There is/was one with a split image rangefinder inside a microprism collar (cat.no. 42215), but the more common ones also have gridlines (cat.42170 and 42217, the last one being the "D" version - these screens were supplied with camera bodies as the "standard" screen in later years.).
It doesn't matter much whether it is an Acute Matte or Acute Matte D screen. The difference between the two is not big.
There were a few pre-Acute Matte screens with split image rangefinder. The best, in my opinion, is the one with only a split image rangefinder (cat.no. 42188). The other one (cat.no. 42218) also has a collar of microprisms around the rangefinder, which i do not like at all.

By the way, the finder equipment of the KIEV 88 cameras, fits also onto the Hasselblad. Even magazines can be easily changed to fit on the HB. If you don't want to spend too much money, this will be a good source.

Cheers

thanks so much..