Adjust the end points to 100/100 and set the total servo throw in RATE mode with the limit pot on the gyro.

After that the end points control your piro rate, not servo throw.

Also the linkage length is just a guide line. If you adjust the linkage so that there is no or little drift in rate mode then heading hold will probably hold just fine.

Make sure you in heading hold mode (solid light)
Also make sure you dont have different rudder trim in different flight modes, any rudder trim will cause the gyro to drift, on the ground flip the flight mode switch and watch the light (engine off) and make sure it stays on solid.