As per IS 455 portland slag cement specification and as per ammendment no 4 of May 2000.
In PSC cement manufacturing, GGBS added should not be less than 25% and should not be more than 70%
Compressive strength of Portland Slag Cement should not be less than 33 Mpa, means PSC available in 33 grade (there are no specification for higher grades of PSC)
So according to IS code you can replace portland cement up to 70% by using GGBS.
You may try replacing cement by more than 70% and share your experiance.