Ramps 1.4 on Wanhao I3 thermocouple reading

[printbus]

I don't think it would explain the large error you are seeing, but when I experimented with a thermocouple board with a MEGA2560/RAMPS setup, I realized there's a fundamental flaw that can lead to at least some error.

The AD595/AD597 sensors provide an output that is fairly stable as supply voltage to the chip varies. This would normally be good. The problem is that full-scale on the analog-to-digital converter in the ATMEGA2560 chip is hardwired to the 5V VCC for the board. At least with Marlin, VCC is assumed to be EXACTLY 5.00 volts. If your VCC is off by 5%, it will lead to your thermocouple readings being off by 5%.