The mere fact that you're trying to calculate a conversion from feet to metres in SEVEN (!) places of decimals (micrometers) while discussing differences between QFE and QNH in the order of six feet (two metres) tells me you really don't have a realistic grasp of what it is you're trying to do.
Fella, your altimeter doesn't/can't read to an accuracy better than 30-50ft or so - why on earth are you consideriing micrometers?