• 21.1 baseline — 0.6 × season 21.6 (18 gm) + 0.4 × last 10 20.3
• −1.3 opponent leak — they concede 345.4/game vs a 369 league average
• −8.1 at this ground — averages 13 there (9 gm)
• vs them: averages 20.4 over 5 games — evidence, never summed

Most signals here point under while you're on the over — worth reading the working above before you commit.

Derived, not measured — plain arithmetic over his real games, shown in full. Adjustments can overlap (a game at this ground can also be a wet one); they add simply, and this note is the admission.