By how, do mean what to say in response to the offer?
I usually politely explain why I can’t accept an offer, and come back with what I would accept. You might say something like “given the amount of time put into each track, along with my expenses for hiring talent and buying instruments, software, hardware and etc, I don’t feel 25% of licenses with no upfront fee is adequate compensation. Would you consider….”
Usually the answer from the client/library is, “Well, we have expenses also. Office expenses, telephone, staff and etc”.
To me, the expenses for either side is a wash, and 50/50 is the only deal that makes sense.