iain
you don't need to be precise, and i doubt that you would break one off or strip the head without throwing your back out trying to crank them down. you should tighten them enough to feel the new gaskets on the plugs crush slightly. it's pretty subtle, as much as you don't want to strip the threads, you don't want them to leak or fall out either.
if you did strip the threads or break one off, you can have the head helicoiled. the have a kit that puts down new threads. its not super expensive. removal of a busted plug isn't too difficult either.
so snug them up tight, crush the gaskets and sleep at night