Do you want to round the start and end times to the nearest tenth of an hour and calculate the difference, or calculate the difference then round it?..... there's a difference.

If it's the latter and you want the result as a decimal, i.e. 8.5 for your example use the formula

=ROUND((A2-A1)*24,1)

If you need to accommodate shifts which start one day and end the next, e.g. 22:00 to 06:00 then change to

=ROUND(MOD(A2-A1,1)*24,1)