Using provided data with linear regression model, the best estimate I get is:
p = int( 0.145188124 + d*0.004538102 + po*0.001904938 + r*0.000925993 )
where:
p - rep power
d - number of days since registration
po - number of posts
r - reputation received
int() - function rounding number down to an integer
Last column shows data (and deviation) calculated using above formula.
Only data of avatars who provided full date of joining was used in calculations.
I'm sure somebody can do better job than that.