The mirin and sake together make up one third of the total recipe volume. Use all mirin, all sake, or half mirin and half sake — all three work. Mirin is sweeter and thicker; sake is drier and more savory. Half and half is a great middle ground if you have both on hand.
Low and slow is the key — a rolling boil will scorch the honey. After 30 minutes the sauce will coat the back of a spoon and deepen in color. It thickens a bit more as it cools, so pull it off the heat before it looks fully done.
One batch (1½ cups) covers both teriyaki nights exactly — ¾ cup marinates and glazes the chicken thighs, ¾ cup glazes the salmon. Make it Sunday afternoon while you linger over a cup of tea, and that's two meals nearly half done for your week!