I am looking for information that defines Principal Residence Exemption when also using home for business.
At what percentage of business use are you required to prorate the PRE
There is no hard rule. This is discussed in detail in other threads on this forum. Basically, the higher the percentage business use, the more likely CRA will question it. But, even if CRA questions it, that doesn’t mean they will deny it. And even if they deny it, that doesn’t mean they are correct - you can always file a NOO to have it looked at more closely by CRA’s tax lawyers, and/or take them to court over it.