Don't worry about 45-degree angles, the best way to pour a can of Guinness into a pint glass is to open the can and immediately place it upside down in the pint glass. The beer won't overflow, and after the bubbles stop foaming, you can gently remove the can and have a perfectly poured pint as shown in the video from culinary weblog Chow.com.
The video in the link below from Chow demonstrates this perfectly. Unfortunately, the video format doesn't work with our software so I can't embed it here, but in another day or so it should be up on the Chow YouTube page and I'll update this page. Until then, please visit the link below.