diff --git a/src/cross-chain-order/cross-chain-order.ts b/src/cross-chain-order/cross-chain-order.ts
index 177d2c2..f5eb0f0 100644
--- a/src/cross-chain-order/cross-chain-order.ts
+++ b/src/cross-chain-order/cross-chain-order.ts
@@ -330,10 +330,15 @@ export class CrossChainOrder {
         )
         const partsCount = this.escrowExtension.hashLockInfo.getPartsCount()
 
+        const calculatedIndex =
+            ((this.makingAmount - remainingAmount + this.makingAmount - 1n) *
+                partsCount) /
+            this.makingAmount
+
         if (remainingAmount === fillAmount) {
-            return Number(partsCount)
+            return Number(calculatedIndex + 2n)
         }
 
-        return Number((partsCount * (fillAmount - 1n)) / this.makingAmount)
+        return Number(calculatedIndex + 1n)
     }
 }