I have to suggest that most of the designs I knew that others did, and that I did, weren't by guess, and certainly weren't by God. At least none of us would ever have even been mistaken for Him.

We often did start with a known and work from there. For example, I did a 2M thermal glider that went through a number of versions. And I used the formulas to give me some numbers to keep in mind while I flew the suckers and found out what they did in practice. The numbers then sort of helped when the next version was drawn up.

One problem you're going to possibly brush against with gliders is your tail size. They wind up being rather small the farther back you place them. The small that matters is the Reynolds "small". That is, the chords we're playing with can actually affect the way the glider handles if you get it too short. And the formulas help you deal with stuff like that.

If you're just looking for a weekend warrior, not a competitive bird, don't stress over any of it. Go with what looks right to you.