Well, to each his own. I wouldn't do it that way, but we all know how weird I am.
In real practice, the only pair of shrouds that starts on one side and ends on the other is the odd-numbered one (assuming there's an odd number of shrouds on a given mast). The others are in pairs, up and down on the same side.
Some books do say that odd-numbered shroud should go all around the masthead and down the other side. Others say it should consist of two separate ropes, port and starboard, cut-spliced together with the splice going over the masthead. On 1/100 scale the difference probably isn't worth fussing over.
In a real ship, prior to the very late nineteenth century, one feature of the rigging is the big stack of loops over the masthead, just above the top. Those loops get special treatment (at least in British and American practice). They're wormed (a lighter line is wound around the big rope to fill in the spaces between strands), parcelled (a strip of canvas is wrapped around the bight, making it noticeably thicker), and served (finished off with a tight, spiral-wound layer of light line). Then the whole thing is covered with tar. The result is a stack of heavy loops that goes a third or even halfway up the masthead.
This is one reason why experienced modelers hate those "pre-formed ratline assemblies." There's just no way to set those...things...up that reproduces the big pile of loops.
On my little model of the frigate Hancock (which is, I'm embarrassed to say, my most recent square-rigged ship) I imitated the appearance of the wormed, parceled, and served shroud loops by giving the loop a thick coating of artist's gesso just before putting the loop over the masthead, then painting it off-black when it dried. I'm pretty happy with the result.
The procedure I generally use goes like this. 1. Seize a deadeye into one end of the line that's going to be the first two shrouds. 2. Test fit the shroud up through the lubber hole and around the masthead, and mark where the loop needs to go. 3. Make the loop, by seizing the shroud to itself. Make the loop tight enough that it barely goes around the masthead. 4. Drop both ends of the shroud pair (one with the deadeye on it, the other without) through the lubber's hole and bed the loop firmly on the masthead. 5. Seize the other deadeye into the second shroud.
Then go on to the other side.
The temporary stay running forward from the masthead is a good idea - up to the point. I don't worry if the mast leans a tiny bit backward when the shrouds are done; then I can tighten the shrouds a little by setting up the stay really taut. (That's how it was usually done in real ships.
In my experience, one of the most challenging jobs in rigging is getting all those deadeyes lined up in a nice, straight line. That means judging the tension on the shrouds so they're all consistent. I know of no guaranteed trick for doing that. (Some folks make simple jigs to hold the deadeyes the right distance apart. I haven't tried that.) What makes particularly difficult is that it has to be done quite early in the rigging process, and in my case that means my fingers are badly out of practice. I usually end up sacrificing more than one pair of shrouds because I didn't get them tight enough.
The good news is that when you're done with all that you'll have a mighty impressive start for your standing rigging. Dave, you've done this several times before; I think you'll get most, if not all, your shrouds right the first time.