Royalties for designing apparel

I’m hoping maybe the Straight Dope might come through where other more topic-specific forums haven’t:

I do graphic design for a smallish company, who recently obtained a licensing agreement with a major nationwide retailer. My company gets a small percentage of net sales from the retailer. One thing my company will be doing is apparel. I designed several mockups that the retailer liked, so my company has offered me to create a line of designs on my own time, and will pay me a royalty % of their take. We will be working out an agreement shortly. Does anyonehave any what a fair % would be to ask for?