common percentage on royalties in optical market / Eyewear

Dear members of Core77,

I am a Productdesigner from Germany and will soon design new glasses.
Could anyone give the information, how high a common percentage on royalties / licence-contract-rate for the Designer in the optical market (Eyeywear-Design) can be?
Is the percentage in the optical business same range as for e. c. furniture-design (2%-5% on sales amount)?

I thank you very much in advance for your help and feedback and stay at your disposal.