Basically your surface feet per minute decreases to zero at the cutting tip. In other words, use the standard formula RPM/3.82 X Cutter Diameter, but instead of the actual cutter diameter, you can approximate the diameter at the actual point of contact of the tool. If you are using the absolute tip, the diameter approaches zero and that is why you will see rubbing and tip degradation. There are some ways around this. First, if you have the opportunity, approach the surface from the bottom rather than the top. This will move the point of contact out from the tip, increasing the SFM. Second, most shops that are doing a bunch of surfacing, use tools other than traditional ball endmills. For instance, any sort of bull endmill. There is also a growing trend to use barrel tools for steep surface and large radius lense tools for shallow surface. These have a twofold advantage of increasing the SFM and increasing the effective radius to produce a finer surface finish with a larger stepover. Mastercam (and I'm sure others) has done a good job of using these new tools in their multiaxis surfacing routines.
Lastly, when the opportunity presents itself, I like to rotate the part being machined in the A axis by a couple of degrees. This prevents me from machining with the actual tip of the cutter, and pushes the point of contact out towards the major diameter increasing the SFM even on low angle surface. This is free, but only applicable in 4 and 5 axis mill work. On my 3 Axis machines, I tend to use bull endmills. As far as actual feeds and speeds, I can't help you with that material, and stepover is very dependent on the type of toolpath you are running in the cam system (scallop, waterline, parallel....)