You may have to sacrifice a couple rolls of film to testing. Pick a typical scene, set the Nikon to 1/4 sec. Shoot film of identical scene in both cameras but bracket the Hassy by 1/3rd stops. Develop in the same tank and contact print. If you find the 2/3 bracket is the one that matches the Nikon, you can adjust your "asa" accordingly and you should get identical exposures afterwards. I used to use a twin camera platform with my Mamiya Universal back in my Velvia days with great success letting my FE do all the metering work.