For FL equivalence, the flaw is the use of the diagonal distance...some formats are square, some are overly long (135, for example), and some approximate the aspect ratio of the 8x10" standard. It is better to consider the vertical dimension of the frame, to calculate 'equivalence' in FL, as this gives the exact same vertical Angle of View regardless of format size!
For 135 format:
- 35mm 'wide angle' is 1.46x vertical,
- 24mm 'very wide' is 1x vertical,
- 100mm 'portrait' is 4.2x vertical.
- 'normal is arguable. 135 format uses a longer FL than its diagonal would lead you to use...45mm. 'Normal' in 135 format have ranged between 45mm - 58mm!
Now apply all those multipliers to the 56mm vertical of 6x6 medium format
- 'wide' is 80mm (81.7)
- 'very wide' is 56mm
- 'portrait' is 235mm
- 'normal' vs a 45mm 'normal in 135 format would be 105mm
Those results above are not what one expect to use, yet they are driven by our expectations arising from using 135 formal SLRs.
So, if you like to carry a 135 format camera with 35mm 'WA' lens around, as many photojournalists liked to do, 80mm would be perfect.
If you prefer the somewhat long FL as 'normal', such as used by typical 135 format SLR, really you want 105-110mm FL