Jeez, this is going to be a pain.
Doesn't have to be. All you need is a set of spacers with which you can set a straight edge and trim the hinge side of the door precisely using a circular saw.
You can make these spacers in about 10 minutes. You could make them out of 1/4 luan plywood or out of some kind of plexiglass or other plastic product. If you make them out of luan you will probably later think they are scrap and throw them away. If you make them out of plexiglass you will have them for later projects when you need to make a precise cut with your circular saw.
Let's pretend that you have a strip of 1/2 or 3/4" plywood about 3 or 4 foot long and 3 or 4" wide laying around. Let's also pretend that you have a couple of old wooden saw horses. If you don't, build a set. You will need them later to lay the door on to trim it.
So lay your strip of plywood on the saw horses and tack or screw it down. Cut off about 1 foot. Take the cut off piece (the one that fell to the floor) and screw it to the piece on the saw horses at about 90 degrees, about 6" from the end. Doesn't have to be exact. About 90 degrees and about 6" is "good nough for what it's for".
This will be your sawing jig for cutting your spacers.
Zip down to Home Depot or Lowe's and purchase the smallest piece of plexiglass, Lexan of what ever they have. Thickness doesn't matter. Can be from 1/8 to 3/8" thick. Just need to be relatively stiff. Let's call this your plastic stuff.
Also purchase a roll of double sided tape. You won't be able to hold your plastic stuff to cut it so you will tape it to your cutting jig with the double sided tape.
Also stop by the garden department and buy a rose bush, azalea plant or hanging basket for the wife. Maybe she won't notice that you spent money for something you wanted.
When you get home give the wife her hanging basket. Tear of a couple strips of double sided tape and stick them to your saw jig. Pull of the backing of the double sided tape and stick your plastic stuff to the saw jig.
Using your circular saw, with the wide side of the saw shoe against the edge guide, cut your plastic stuff. Set your depth of cut to just a little more than the thickness of you plastic stuff. That is the angle the tooth of a circular saw blade likes best.
After making the first cut take the piece that fell to the floor and put it somewhere where you can find it later when your neighbor ask you to make a set of spacers for him.
Take the piece stuck to the saw jig and turn it 90 degrees and stick it down again. Cut it again, this time using the narrow side of the saw shoe against the edge guide. The piece stuck to the saw jig this time, will be a spacer. The length of which will be exact distance between the saw blade and the wide side of the saw shoe, the width of which will be the exact distance between saw blade and the narrow side of the saw shoe.
Take the piece of your plastic stuff that fell to the floor this time and stick it to your saw jig again. Cut again, again using the narrow edge of the saw. You now have a set of spacers with which you can set a straight edge to make a precise cut with a circular saw.
To use your spacers, measure your door top and bottom where you want to cut. Clamp your spacers to the door on the marks, with small spring clamps. Place your straight edge against the spacers and clamp in place. Remove the spacers and make your cut with your circular saw.
You can use the wide side of the saw shoe against the edge guide (always best if you can) or you can use the narrow side of the saw shoe against the edge guide (because sometimes you have to). All depends on how you use the spacers.
A smart person would also mark the location of the hinge mortise before trimming the hinge side of the door.
