Calibration of tach

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.

The nice thing about using an optical tach is you can check it accuracy by pointing at an incandescent light bulb. 60 hertz system should read 3600 rpms. I know small engine guys use tachometers but again accuracy may be an issue but may be worth checking it out.

Now I feel like a complete idiot. Hand held tach is what I need. Thanks for filling me in. 

No computer. Tiller model. I only estimate the “ 250 rpms difference” to explain my situation. If I am going to adjust, I want to be exact. I have never heard of a hand held tach... I will look into that.

I take it there is no computer on that motor so I would try and locate a hand held tachometer with a piece of reflective tape on the flywheel.

While on the water with a 1/8" flat head or small Phillips screwdriver turn the adjustment screw 180. Take it for a spin and keep adjusting from there. If like me, it means keeping under the console clear so you can get on your back with a flashlight or headlamp. My Lund new from the factory was backwards...