I don't know about standard percentages, but I do know it is usually of the gross for referral fee to avoid any confusion or discrepancy as to the profit. You don't want people arguing over what job costs are and whether something should be put into a cost for a certain cake or spread over several cakes (say you need a new pan for a cake that you purchase and take out of the profit for that cake to which you pay a referral fee, but then you have a new pan you can use for other cakes and it wouldn't come out of the profit for those cakes which you may or may not be paying referral fees on.)
Take for example a franchise (like McDonald's) in order to use the resources, name, training tools, etc. each franchise pays McD's a % of their gross, not net.
I am using super vague examples and guidelines as I do not have any contracts for referrals, but know of people that do for different industries.
Also, if you do have a contract for referral fees payment for the referral should ALWAYS be after you receive payment in full from the client, never up front.