Yes, but it is quite a bit more accurate, and faster, if you are doing a LOT of doors to get yourself a cheap mortise and marking gauge (look for Draper #64458 - £12 to £15). You set it approximately at the half way point then mark a dot from one side, then the other. If the second dot is at the same point as the first, you have found the centre. If not you need to adjust the head of the gauge by 1/2 of the difference between the two dots, then try again (until you get it right).
The other side of this device has two pins, one fixed, one adjustable (on a slide), which allows you to set the two pins at the exact width of a piece of hardware (set directly from the item), such as a lock body or a face plate. So by using the same technique of "dotting" from both sides of a door and adjusting to centre the pins on the door, you get a marking tool which will allow you to simultaneously and accurately mark out the edges of your lock face plate accurately and repeatably (and with nary a tape measure or ruler in sight). Try it.
You scribe the top - and do it before marking and chopping out the hinge recesses. You must aim to get a visually consistent gap around all three sides of the door, even I'd that means a bit of judicious planing.
The bottom is a bit different. Fire doors need to be set with a gap no greater than 4mm from the floor at any point when the door is closed. For non fire doors this does not apply but aesthetics dictate that gapping should be consistent whilst also permitting the door to open - so sometimes compromises are needed
Something I do a lot, especially on flush (veneered) doors, but I'll give you another one, then: if the door is veneered (think flush doors) and is tight to a carpet especially a shag pile one, it is often worth putting a 1mm or so chamfer on the bottom. This reduces any tendency for the pile to snag on the veneer and chip it off at the bottom of the door