Employee Timesheet; calculating +/- hours worked

This formula in G4 filled down will give +/- values in decimal format when formatted as general.

Formula:

=IF(B4="","",INT(F4)*24+HOUR(F4)+ROUND(MINUTE(F4)/60,2)-8.5)

I don't understand the language so the values in column H I can't help with.

Format F37 as [h]:mm