Which side of reading if it is towards junction 12 side and if you are doing it in rush hour, i would perhaps suggest the following: go down to end of the iffley rd to the ring road and take the road that is down the side of heyford hill sainsbury's. Take this road cross country via wallingford and pangbourne then the a4 to junction 12 m4
or over donnington bridge, turn left, get to the ringroad then a34, M4 to junction 11/10 depending on where in reading