I still see some Spey lines with weight designations offered on the box. Presently, I'm in the same boat with a new build of a 16 footer in hand.
My solution will be to attend a local Spay Clave where the manufacturers show up with all their products, and one can walk to the river and give each a try for a hour or so at no cost. Not aware of any such event, however, in your neck of the woods. If one can find contact info for the actual blank manufacturer, they will probably have some grain window information for you.
The Scandi and Skagit setups are fine and rigged such that one can switch among a variety of tips (heavy sinkers to floaters) to match conditions. They are head systems, designed mostly for distance, and one also needs some running or shooting line behind. I'd be inclined to line up for the heavy side of one's rod designation. If things are too heavy, then one can gradually reduce the lengths of his heads until things come into balance. These would likely offer the greatest versatility if one is going off into unknown waters and conditions. Their downside is that one can do very little effective mending once his fly is in the water.
If one's into mostly shallow water fishing, then I tend toward long belly super Spey lines or even floating double tapers. I don't like to waste time striping in line between casts. The whole idea of Spey rods is that one can handle fishable lengths of line with minimal manipulations.
If there's anyone at all around with some lines, I'd also suggest trying their setups on your rods. Usually one begins casting, gradually lengthening his line until suddenly the system seems to come alive. At that point, make an attempt to determine the weight (grains) of the line/leader that was outside of ones tip top. That would be a good starting point if one is making purchases based solely on grain weight.
Also, if one is not truly an adept Spey caster, he might round up a local authority that can really give the rods an effective test. Spey casting is easy, but not for those that have 30+ years of single handed work behind them. Newbie's learn faster than experienced single handers.
One might also go to a Spey forum and pose the same question mentioning the brand and all possible specs for his rods. There's a good chance someone has handled similar makes and models, and they can offer authoritative advice.
Here's a link to a forum I frequent: Spey pages link
Here's a discussion of your issue on that same forum. Line designation link
If at all possible, do try to find a source where one can give things a test run. At retail, one can easily tie up $100 to $175 for a Spey line and that's a bummer if one is not happy.
Good luck with your builds and keep us up on your doings,