How DRY & expressive can we get by making the subject a "let"-defined helper?

bowling_game_spec.rb

describe BowlingGame, '#score' do
  subject { score }
  let(:game) { BowlingGame.new }
  let(:score) do
    rolls.flatten.each { |roll| game.roll(roll) }
    game.score
  end

  context "when there are no spares or strikes" do
    context "all gutter balls" do
      let(:rolls) { [0] * 20 }
      it { should == 0 }
    end

    context "a 5, and 19 gutter balls" do
      let(:rolls) { [5, [0] * 19] }
      it { should == 5 }
    end

    context "0 through 9, rolled twice each" do
      let(:rolls) { [0,1,2,3,4,5,6,7,8,9,0,1,2,3,4,5,6,7,8,9] }
      it { should == 90 }
    end
  end