Not knowing a formula outright, I decided to follow the units of some relationships I did know. Radiation pressure is defined as force per area and also intensity divided by velocity (the speed of light here of course). Breaking intensity down into power per area and isolating force gave me the relationship F=(Power/Velocity),where power is given and the velocity is a constant.
My work is in the attachment, where I double checked the units too, comment with any questions.
