Personally, I use FSWizard online for all my feeds and speeds calculations and I've found it to work quite well.
http://zero-divide.net/?page=fswizard
I've done parts out of 3/16" 6061 aluminum plate, with a .25" 3-flute carbide endmill, 10% stepover HSM toolpath, 6000 RPM, and 144 IPM.
I'm setting up today to do our gearbox plates from 1/4" 6061 aluminum plate, with a .25" 3-flute carbide endmill, 25% stepover HSM toolpath, 6000 RPM, and 29 IPM.
Both cases have flood coolant, and both took the full thickness of the part in one pass. What I haven't tried is comparing the job time between say a 10% stepover and a high feed, and a 25% stepover and a lower feed. That would be interesting to see which is faster.
In aluminum, you're limited really only by your spindle RPM (for smaller cutters). Next machine I get will be 10k RPM.
As mentioned above, avoid slotting, and avoid straight plunging wherever possible The tool does not like either of these much, specially straight plunging.
What machine do you have?