Personally, having hung hundreds, (if not a couple of thousand or more...) doors over the last 30 years, if the linings are as far out as Seamaster says, then its unlikely they will be straight either, so I wouldn't consider the straight edge and circ saw approach.
I was taught with handplanes, (and have taken 10mm off with them on many occasions before powertools became popular!) and the principle is to shoot the hanging edge in first to ensure the back edge fit is uniform and won't bind, ie. it has to be shot tight to accomodate for bows or hollows, giving a uniform gap once hung. I would use an electric planer, using the same principle as hand planing, so making simple marks to show where the timber needs removing. Once its close, I'll clean up with a handplane (4 or 4.5 smoother is my favourite) to take the planer block ripples out. Do enough of them and you get an eye for how much is needed off, and from where.
The head I always keep full, if it needs timber removed to get over a head thst isn't level/square, then i'll shoot from nothing to the amount needed, set with a bevel, so the majority of the waste is from the bottom, keeping the heads looking as close to each other as possible.
Any cutting for height I'll do from the bottom rail if possible, using a circ saw if there is enough to be removed easily.
Depending on your own experience and ability, you could hang the door and swing it, and let the closing edge clip, strike a pencil up the stile and shoot back in, but I usually try to fit first time :shock: so I have the penny gap all round allowed for, and then hang, hopefully fitting first time once hung, with all screws in on the door stile and a single screw in each hinge leaf on the lining to check. If there is a slight clipping as it closes, this usually solves itself once all the screws are in as the leaves pull back tight to the lining.
Most electric planers can remove 5mm in two passes, far quicker than setting up a saw and guide in my experience. It also allows you to remove more waste in the areas where needed, fine tuning the depth of cut if needed, plus, if you are confident enough, you can also take the back bevels off for closing at the same time.
As for heights of door linings, older houses tend to be little over 6'6in as the floor coverings, if any, were linoleum, or simply waxed boards.
Nowadays, with carpets and underlays, linings can be as much as an inch oversized to allow for it.
Hollow doors are a pain if they have to have too much removed, i've had to re-glue strips back in on many occasions...
Hope this helps rather than confuses...
Andy
Andy