Printing shorter than it thinks

So, i have finally dialed in my Folger tech Prusa i3 to a point where only a few issues remain. However, these issues are wreaking havoc on my prints and are major pain in the butt.

PROBLEM ONE: the print is coming out pretty well but there always shorter than they should be. e.g a 25mm test cube will come out at only 20mm tall.

PROBLEM TWO: it prints absolutely miserably with over hangs. any bridging bast 5mm in length will simply amount to a frustrating, goopy pile of sadness and using any supports whatsoever results in them being humanly impossible to remove without harming the print.

PLEASE HELP.
thanks.

1. Test whether all Z movement is 20% too small. If it is, increase the Z steps/mm in the firmware config by a factor of 5/4.

2. If you are printing PLA, install a print cooling fan to get better bridges and overhangs.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

In the config there it gives me only these options
would altering the mm/sec (in red) do the same thing?
thanks in advance.

#define DEFAULT_AXIS_STEPS_PER_UNIT {80,80,3080,305} // default steps per unit for Ultimaker
#define DEFAULT_MAX_FEEDRATE {500, 500, 4, 25} // (mm/sec)
#define DEFAULT_MAX_ACCELERATION {9000,9000,180,10000} // X, Y, Z, E maximum start speed for accelerated moves. E default values are good for Skeinforge 40+, for older versions raise them a lot.

Edited 1 time(s). Last edit at 08/05/2015 10:42AM by krimsonek.

UPDATE: THANK YOUOUOUOUOUOU SOOOO MUCH. all i had to change was 3080 to 3080*5.2/4 NOW IT WORKS PERFECTLY
THANKS!