If using a tape measure it's best to stand against an open door - get a rough idea of where your head is on the door and sellotape a piece of paper there then use a pen or pencil stand against the door back straight feet together and mark a line across the top of your head then measure from the ground up to the line it should be pretty accurate.
Mistakes people make is that they stand against a wall with a skirting board around it, or they don't put their feet together or they stand on thick carpet and only measure to the top of the carpet etc.