diff --git a/src/services/projectionChallenger.js b/src/services/projectionChallenger.js
index 449d1b9..39210d3 100644
--- a/src/services/projectionChallenger.js
+++ b/src/services/projectionChallenger.js
@@ -57,6 +57,13 @@ const abbrOf = (team) => {
   return NAME_TO_ABBR[s.toLowerCase()] || null;
 };
 
+/** American odds → implied probability (vigged). Null on absent/zero. */
+function americanToImplied(odds) {
+  const o = num(odds);
+  if (o == null || o === 0) return null;
+  return o < 0 ? (-o) / (-o + 100) : 100 / (o + 100);
+}
+
 /** Park factor (multiplier ~1.0) for a team's home venue, or null. */
 function parkFactorFor(teamAbbr) {
   if (!teamAbbr) return null;
@@ -179,13 +186,19 @@ function projectProp({
   const rungs = dist.ladder(nb, LADDER_MAX);
 
   // ── LADDER + book implied (only the traded rung has a book number) ────────
+  // The BOOK's implied probability comes from the book ODDS (grades carry
+  // book_odds, not a de-vigged fair_prob). This is the vigged implied — a known
+  // constant offset the ledger measures on both sides — expressed on the OVER
+  // basis so it's directly comparable to our P(≥ rung).
   const tradedRung = isNum(line) ? Math.max(1, Math.ceil(line)) : null;
-  const fairProb = num(grade && grade.fair_prob); // de-vigged, graded direction
+  const bookOdds = num(grade && (grade.book_odds != null ? grade.book_odds : grade.locked_odds));
+  const rawImplied = americanToImplied(bookOdds);
+  const bookOverImplied = rawImplied == null ? null
+    : Math.round((direction === 'under' ? 1 - rawImplied : rawImplied) * 1000) / 1000;
   const ladderOut = rungs.map((r) => ({
     rung: r.rung,
     p_at_least: r.p_at_least,
-    book_implied: (tradedRung != null && r.rung === tradedRung && direction === 'over' && fairProb != null)
-      ? Math.round(fairProb * 1000) / 1000 : null,
+    book_implied: (tradedRung != null && r.rung === tradedRung) ? bookOverImplied : null,
   }));
   const pOverLine = tradedRung != null ? dist.round3(dist.nbSurvival(nb.r, nb.p, tradedRung)) : null;
 
@@ -194,7 +207,7 @@ function projectProp({
     proj_point: point,
     proj_line: line,
     proj_p_over_line: pOverLine,
-    proj_book_implied: (direction === 'over' && fairProb != null) ? Math.round(fairProb * 1000) / 1000 : null,
+    proj_book_implied: bookOverImplied,
     proj_distribution: {
       family: 'negative_binomial',
       r: dist.round3(nb.r), p: dist.round3(nb.p),
diff --git a/tests/unit/projectionChallenger.test.js b/tests/unit/projectionChallenger.test.js
index 3ce96fd..3ae91a5 100644
--- a/tests/unit/projectionChallenger.test.js
+++ b/tests/unit/projectionChallenger.test.js
@@ -76,7 +76,8 @@ describe('projectProp — full object, never abstains, plausible', () => {
   const hitLog = (vals) => vals.map((h) => ({ isHome: true, opponent: 'Boston Red Sox', stat: { hits: h } }));
 
   it('emits distribution + full ladder + point + factor breakdown', () => {
-    const grade = { stat_type: 'hits', line: 0.5, direction: 'over', season_avg: 0.9, fair_prob: 0.62, team: 'NYY' };
+    // book_odds -150 → implied 0.6 (the book's number the grade actually carries)
+    const grade = { stat_type: 'hits', line: 0.5, direction: 'over', season_avg: 0.9, book_odds: -150, team: 'NYY' };
     const p = pc.projectProp({ grade, gameLog: hitLog([1, 0, 2, 1, 1, 0, 1, 2, 1, 0]) });
     expect(p.proj_version).toBe('proj-v1');
     expect(p.proj_distribution.family).toBe('negative_binomial');
@@ -85,11 +86,19 @@ describe('projectProp — full object, never abstains, plausible', () => {
     expect(p.proj_factors.breakdown.map((f) => f.label)).toEqual(
       expect.arrayContaining(['park_relative', 'weather', 'platoon', 'matchup']),
     );
-    // traded rung (ceil 0.5 = 1) carries the book-implied for comparison
-    expect(p.proj_ladder.find((r) => r.rung === 1).book_implied).toBeCloseTo(0.62, 3);
+    // traded rung (ceil 0.5 = 1) carries the BOOK-implied (from odds) for comparison
+    expect(p.proj_ladder.find((r) => r.rung === 1).book_implied).toBeCloseTo(0.6, 3);
+    expect(p.proj_book_implied).toBeCloseTo(0.6, 3);
     expect(p.proj_p_over_line).toBeGreaterThan(0);
   });
 
+  it('book implied is expressed on the OVER basis for an under prop', () => {
+    // under at -150 (implied 0.6 the ball stays under) → over-basis implied 0.4
+    const grade = { stat_type: 'hits', line: 1.5, direction: 'under', season_avg: 0.9, book_odds: -150, team: 'NYY' };
+    const p = pc.projectProp({ grade, gameLog: hitLog([1, 0, 2, 1, 1, 0, 1, 2, 1, 0]) });
+    expect(p.proj_book_implied).toBeCloseTo(0.4, 3);
+  });
+
   it('NEVER abstains — an empty game log still projects (wide, from the prior)', () => {
     const grade = { stat_type: 'hits', line: 0.5, direction: 'over', season_avg: 0.9, team: 'NYY' };
     const p = pc.projectProp({ grade, gameLog: [] });