If you are unsure, it is best to use 25 degrees. This seems to be the generally accepted standard angle. General:
A smaller (more acute) angle gives a sharper cutting edge, but also becomes dull again more quickly.
A larger (duller) angle gives a duller cutting edge, but the sharpness lasts longer (tool life). So if you are cutting mainly softwood, you could grind 20-25 degrees, for hardwood 25 - 35. With plane irons you can often find larger angles over 30 degrees. Of course, only if the plane geometry plays along. Basically, I don't think the exact angle is that important. However, it is important for resharpening that you can easily repeat the last angle used without having to adjust for a long time.