Scala Interest calculator based on bands, rounded to precision

InterestCalculator.scala

case class Band(top: Double, interest: Double)

class InterestCalculator(bands: List[Band], precision: Int = 2) {

  def calc(amount: Double) = {

    val interest = bands.collectFirst {
      case Band(top, interest) if top >= amount => interest * amount
    }.getOrElse(0.0)

    round(interest)
  }

  def round(d: Double) = {
    val multiplier = math.pow(10, precision)
    math.round(d * multiplier) / multiplier
  }
}

InterestCalculatorSpec.scala

import org.scalatest.{FlatSpec, Matchers}

class InterestCalculatorSpec extends FlatSpec with Matchers {

 val calculator = new InterestCalculator(List(
    Band(0.0, 0),
    Band(1000.0, 0.01),
    Band(5000.0, 0.02),
    Band(Double.PositiveInfinity, 0.03)
  ))

  "interest-calculator" should "work for £0 to £1000" in {
    calculator.calc(0) shouldEqual 0
    calculator.calc(1000) shouldEqual 10
    calculator.calc(500) shouldEqual 5
  }
  it should "work for fractions" in {
    calculator.calc(1) shouldEqual 0.01
    calculator.calc(13.2) shouldEqual 0.13
    calculator.calc(12.6) shouldEqual 0.13
  }
  it should "work for less than £0" in {
    calculator.calc(-800) shouldEqual 0
  }
  it should "work for £1000 to £5000" in {
    calculator.calc(3000) shouldEqual 60
    calculator.calc(5000) shouldEqual 100
  }
  it should "work for £5000+" in {
    calculator.calc(20000) shouldEqual 600
  }

}