Since you started from a solid ground zero, roundness should become increasingly stable as tension mounts. Continue adding small layers of tension and patient corrections until the wheel feels as tight as a known good wheel of similar design. You're venturing beyond maximum desirable tension. Loosen each spoke a full turn or so before trying to finish the wheel. Also, keep an eye on the rim, not just spokes, to spot overtensioning.
Some rims give distinct signs, such as puckering at the nipple, to indicate maximum tension.
Do not try to make too many corrections at full tension. A fully tightened wheel resists change, which is why it is so durable on the road. A tension-balanced wheel is remarkably stable. It does not need the prestressing or overstressing often applied to lesser-built wheels. Stressing tries to prevent 2 types of problems.