fix: Fixes failing static pressure tests

Tests/EstimatedPressureTests/EstimatedPressureTests.swift

@@ -65,7 +65,38 @@ final class PositiveNumericTests: XCTestCase {
     )
 
     sut = envelope.externalStaticPressure.positiveValue
-    XCTAssertEqual(round(sut * 10) / 10, 0.7)
+    XCTAssertEqual(round(sut * 10) / 10, 0.5)
+
+  }
+  
+  func testExternalStaticPressureWithManufacturersFilterDrop() {
+    var envelope = EquipmentMeasurement.furnaceAndCoil(
+      .init(
+        airflow: 1200,
+        manufacturersIncludedFilterPressureDrop: 0.1,
+        returnPlenumPressure: -0.1,
+        postFilterPressure: -0.2, // here to test it makes positive.
+        preCoilPressure: 0.4,
+        supplyPlenumPressure: 0.1
+      )
+    )
+
+    var sut = envelope.externalStaticPressure.positiveValue
+    XCTAssertEqual(round(sut * 10) / 10, 0.5)
+
+
+    envelope = .airHandler(
+      .init(
+        airflow: 1200,
+        manufacturersIncludedFilterPressureDrop: 0.1,
+        returnPlenumPressure: -0.2,
+        postFilterPressure: 0.3,
+        postCoilPressure: 0.5,
+        supplyPlenumPressure: 0.3)
+    )
+
+    sut = envelope.externalStaticPressure.positiveValue
+    XCTAssertEqual(round(sut * 10) / 10, 0.5)
 
   }