• 91.3 baseline — 0.6 × season 91.8 (5 gm) + 0.4 × last 10 90.5
• −7.4 opponent leak — they concede 1550.3/game vs a 1686.6 league average
• −12.9 at this ground — averages 78.4 there (8 gm)
• vs them: averages 140.0 over 2 games — 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.