More optimisation.

This ATmega is apparently good at multiplication.

  FLASH : 20232 bytes          142%       66%        32%       16%
  RAM   :  2169 bytes          212%      106%        53%       27%
  EEPROM:    16 bytes            2%        1%         1%        1%

  LED on time minimum: 1646 clock cycles,
  LED on time maximum: 2714 clock cycles,
  LED on time average: 2486,61 clock cycles,