I have a 97 Mariner 60. My old tach failed so I purchased a new one and i estimate it approximately 250 rpms off at WOT. It does have a calibration screw on the back as well as the settings for reading the motor pulses. ( set on 6p as was my original) How would I correctly and accurately calibrate the new tach to the engine? Prior to the new tach my wot was about 250 rpms higher as well as my idle rpms seem to be reading low.