Hi Smile,
it's not a subject that I know too much about but I don't think there is a level that is okay.
I've done a quick search but the sites that I've read just mention medium or hgh as being at risk of DKA but none of them say what level medium or high actually is.
I think the ketostix give readings of 0 or negative, +, ++, +++ etc.
I wouldn't normally check for ketones unless I was ill or I'd tested my BG and it was higher than 14 mmols or so.
If I had a reading of + then I would check my BG again and if it was high I would take measures to reduce it such as a correction bolus.
If the reading was ++ or higher then I think I would contact my DSN, GP or NHS direct for advice.
If you have ketones then it's very important to drink lots of water.