First off, since this is an HR forum, I feel like I've gotta say that using amphetamines as a crutch (unless you have legit ADD/ADHD and "crutch" means "aid to function in society") of any sort is a baddddd idea. I used to work with a guy who abused his script for socialization and work. I don't know his dosages, but I do know that he almost never ate or slept. He's a shell of himself now, totally unproductive, horrible social anxiety, depression etc, probably due to other factors too, but he started out with fairly innocent intentions, was never a true speed-freak. So just be careful, control your use so it can still be USEFUL to you, ya dig?
With that out of the way, dosage really depends on the individual. I have ADHD so my doses are a little higher than the average person needs (I can't feel 10mg).
I use 15mg IR for work/organization, mild social boost. 30mg XR is good for being very productive, not good for more mundane work or organization, hugely social at this dose for me but I kind of dislike that because I talk too much as it is.
So if I were you, I'd start at 10mg IR and see how it goes. If it truly doesn't feel like enough, next time take 20mg. My SO, who doesn't have ADHD, gets approximately the same responses to those doses that I get to 15 and 30mg, respectively.
One last thing, for many kinds of work (that don't require extended focus or organization), I find caffeine to be completely sufficient. It gives me a little social boost too. Most people have an ungodly tolerance to the stuff but I took a year long caffeine break and can't believe how effective it is again.