My two cents worth
In my opinion, there is a simple process to setting up a rig. It's a little iterative, but... simple.
In broad terms, you want to get the mast centered in the partners (or at the step, if deck-stepped), then get the masthead centered over the boat, then get everything "in column" between those two points, and then snugged up to keep it that way.
First, as a check, measure from the mast partners to the chainplate on each side. I've never seen this on an Ericson, but on some boats the hole for the mast isn't centered between the chainplates (!). Not much you can do about it, but good to know while you're tuning the rig. If the rig is deck-stepped, you may be able to move the step to correct this. If keel stepped, you may be able to block the mast at the partners to mitigate it. Either way, you want the mast at the deck to be as centered between the chainplates as possible.
Next step is to center the masthead over the boat. Check this by either raising a long tape-measure to the top of the mast using the main halyard, or using the main halyard itself *if* there is little or no stretch in the halyard. The process is to touch the tape (or the halyard shackle) against the chainplate on either side of the boat. When the distance from masthead to chainplate is the same on both sides, the masthead is centered between the chainplates. at that point you can snug up both upper shrouds equally - snug, but not tight.
Then, sighting up the luff track, check to see that the mast is straight between the gooseneck and the masthead. If the rig has double-spreaders, I find it easiest to work from the top down, but that's just a preference. The main thing is, you want to get the mast "in column", and then snug the lowers (and intermediates, if present) equally so that it stays that way. Check often by sighting up the luff track to be sure it is staying straight as you go. When you're done, snug everything up evenly in pairs (both uppers, an equal number of turns, then both intermediates, etc) so that the wires feel "firm", not "sloppy".
At that point, you have a straight, centered mast at the dock. The only remaining question is how much to (evenly) tighten those shrouds so it stays that way when sailing. The best way, IMO, is to go sailing. Pick a calm day (morning, light breeze, flat water) and put the boat close-hauled on one tack. Check to see whether the leeward shrouds get any slack in them. If they do, snug them up (not tight!), and keep track of how many turns you adjusted each turnbuckle. Then tack, put the boat close-hauled on the other board, and adjust the leeward turnbuckles by the same number of turns. Repeat this process a few times, checking straightness by sighting up the mast track periodically, until the leeward shrouds show little or no softness on either tack. They don't have to be super-tight - don't over-tighten them - they're to hold the mast upright and straight, not sling it through the bottom of the boat like an arrow from a bowstring.
ObNote, while I love Loos gauges for dinghies and small boats, I lose confidence in their consistency as the wire size grows. At some point, the Loos gauge is telling you how stiff the wire is rather than how taut it is. YMMV, but once you get to 1x19 wire of 3/16" or larger, I think the Loos gauge loses much of its utility. Just my opinion.
Note, too, that many of the Ericson owner's-manuals have their recommended process for tuning a rig. The E32 manual, for example (available in the downloads section) walks through the whole process, including a couple of tidbits about relative tension between the forward-lowers and aft-lowers, and putting a small amount of rake into the rig by adjusting headstay and backstay.
Bruce
Edited to add: it's important to check spreader angles as part of the rig tuning process, too. The angles between the shroud and the spreader should be the same both above and below the spreader. In math terms, the spreader should "bisect the angle" that the shroud-wire forms as it bends around the tip of the spreader. This ensures that the spreader is in compression, and not being tempted to slip up or down on the shroud. Often overlooked, but important.
