Correlation coefficient values above 0.9 are considered acceptable in general..

I think I read about the good fit indication with R2 >=0.75 in Douglas Montgomery or other Experimental design and data analysis book, longn a go though. It is principally preferred to try another model if R2 /R2 adj. Values are less. It should be higher than this value to confidently claim that our model describes the association between our variables.

Thanks to all for giving valuables information.