• 23.2 baseline — 0.6 × season 22.9 (19 gm) + 0.4 × last 10 23.6
• +1.3 opponent leak — they concede 389.2/game vs a 369 league average
• −7.6 at this ground — averages 15.6 there (54 gm)
• −6.3 wet forecast — averages 16.9 in the wet (9 gm)
• vs them: averages 14.8 over 6 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.