This calculator will make certain types of SU-8 available in the menu based on your thickness, then use data for your selected type of SU-8 to interpolate a spin speed.
It uses thickness, spin speed, pre-bake times, post-exposure bake times, and development times based off of data available in MicroChem's datasheets, as organized at BYU (much more background information here).
If you want to browse all types of SU-8, refresh the page and don't type into the thickness input window.