i know that a conversion has to be taken in a straight line from where the ball is placed. but i want to know how far from the try line you have to be? thank you

There is no set distance, so long as the convert is kicked from along that line you describe. If you are right between the posts, then far enough out so that there is no danger your kick will be charged down (defenders are allowed to charge during a convert), but no further. Say ten or fifteen metres. However if the ball was grounded near the touch line, then the kicker will move the ball back 20 or 30 metres (or more) in order to improve the angle. In either case, how far the ball is taken back is entirely up to the kicker.