Whats wrong with my prints?

so, i3 pro b, marlin 1.0.0, using pla at 190 and bed at 75. 0.4 mm hot end, 1.75mm pla

i have spent hours levelling the bed, checking, home, recheck... its perfect! 1 sheet of white paper just grabs the hot end, at 5 positions, centre and extremities.

small prints are ok, i say ok, they look right, they print ok, but they are inaccurate measurement wise, my 20mm cube ranges from 19.71 - 20.10 on different edges.

however, my larger prints, rarely get past the 1st layer.

the pla squishes nicely into the bed, and the first line is beautiful, but then the second line is half over the first line. by the time I've got 5/6 lines printed, the hot end is touching the already printed lines and it starts to curl.

if i adjust the bed, to get it closer, i get nearly see through sections (too close)
any ideas?

sounds like your z axis steps/mm are out,
Home Z then send command to move Z up by 100mm then measure the distance that Z actually moved. adjust Z steps accordingly.

that would make sense. I've left the configuration as it came, although it was a diy printer. there must be some variances when following badly written chinglish instructions.

thanks

better bed levelling was the solution!

started from scratch, moved the bed screws to halfway, set the height to a paper thick in the centre, then moved out to the edges and revelled with screw edges and then printing was fine.