Honestly I wouldn't worry about it. I think there's an overemphasis on pitch rate verging on paranoia, especially for ales. I would go with it, you're going to get great beer. If you wanted to get the "recommended" pitch rate you'd need to add 1000 to 1500 ml of starter wort which means you'd need a bigger vessel. Even if you have one you're now adding yet another possibility of infection. I would chalk this up to an opportunity for improvement for next time! Go to Mr. Malty or another pitch calculator next time. Don't be surprised if, even when you get your stir plate back, you find that a 1000ml flask is not big enough for anything but a mid gravity brew.