OK, firstly thank you to Dodgeman for posting this info, very kind of you. Now the theory for obtaining a length for the pushrod when using R.R's is demonstrated in this picture, you will see on the bottom drawing that the cam is rotated to half lift and an imaginary line is drawn from the tip of the valve through the centre of the rocker shaft and picks up on the top of the pushrod, the reason for setting up on half lift is so that the rocker moves equal distances ie, in this case 45deg to full lift and 45deg to no lift which is optimum in a perfect world. This info was shared with me by Rob Tucker (well known in the drag racing circle) and he says this basic formula is what drag racing engine builders try to achieve, some times its not possible to get the "45-90-135" angles, some where close is good enough depending on each motor, having the angles way out can have nasty affects,ie bending pushrods, excessive wear on the pivots on the rockers.
Hope this maybe of some use to some of you now or in the future
Gary