i need a sketch on my arduino uno to start exactly five seconds after i turn on the power.
The approach seems reasonable - the bootloader delay will depend what bootloader you're using but should be fixed as long as you don't burn a new bootloader. How exactly do you need to get the delay? As a starting point you could just write a sketch that writes a character to the serial port and see how much delay there is between releasing the reset button and that output appearing.
Alternatively try to track down which bootloader you are using (it should be possible to work it out), or simply get rid of the bootloader altogether - you'd need to upload using ICSP subsequently.