we bring a point from infinity to a point where we can calculate..cant we put an estimate in any point of infinity? if not then why so?

Technically infinity does not exist. There is always a finite distance howsoever large that can be considered for measurement. However, if we put a rough estimate on infinity then we would be restricting the upper limit of distance. Also, the extent to which potential or electric field, magnetic field, gravitational field, etc can be felt is infinitely large. Hence, defining a large number for infinity would yield correct results but would be logically inaccurate.