| the general answer seems to be if your BG is over 13mmol (230), then you shouldn't exercise.
As Bluesky has stated. If your BG is high then you don't have enough insulin, and there is a risk that you may start producing ketones. Ketones acidify the blood, and if you exercise which dehydrates you, then you can make this effect worse.
However....
I find that I can exercise if I'm high. As long as there isn't another reason for the high BG (illness for instance), then I will administer a correction - I aim to correct to 8 mmol (145) rather than 5 mmol (90) which is my usual. Your muscles still need insulin to be able to utilise that spare glucose.
I also make sure that the exercise is firmly aerobic until my BG is back to normal levels. - no intense anaerobic activity - weight lifting, sprinting, hard hill climbing etc as these can cause BGs to soar. If you are on a machine then you need to keep your heart rate below 65% MHR.
For me from testing and experimenting 15mmol (270) is the highest BG that I can safely exercise with. But I always correct, and always start off gentle.  | 
