• 18.8 baseline — 0.6 × season 18.5 (13 gm) + 0.4 × last 10 19.3
• −0.6 opponent leak — they concede 357.8/game vs a 369 league average
• −3.0 at this ground — averages 15.9 there (29 gm)
• vs them: averages 5.0 over 1 game — evidence, never summed

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.