rather than stretching the springs - it's better to screw the bolts up so the springs are really tight.
The other thing I've done with my i3 is to make some thumbscrews that use locking nuts and screw the springs up pretty tight. That way, calibration should be almost a one time affair.
Or rather it will be once I've fitted an aluminium base - at the moment the bed frame is only attached to the printing frame at one end. Crap design. But once I've bolted both frames to a sheet of 4 or 5mm aluminium, It should be a one time calibration :-)