Sydney,

ShimaNO 7 speed spacing uses:

4 x 3.1mm spacers, and
2 x 3.3mm spacers

ShimaNO 8 speed uses 3.0mm spacers.

Ignoring thickness of cogs (as far as I know I have always used them interchangeably between 7s and 8s):

7 speed total spacers = (4 x 3.1) + (2 x 3.3) = 19mm

8 speed total spacers = 7 x 3.0 = 21mm

The rest of the difference 4.5mm - (21mm - 19mm) = 2.5mm would be explained by the additional cog.

Voila!