Tach reading high

I might not be the best student, but I'm trying lol. What initially was a carb issue that I was wrestling with, then this damned "tach issue". I've gradually worked my way back to it looking like a connection/ supply voltage issue at the ignition switch "area".
With that being said, while trying to diagnose, I noticed that the fusible link I installed while doing Mad Bypass a couple years back was undersized. Its 18 ga now and Mad Bypass suggests 16ga. I initially ran the 18 ga from starter relay to under the dash where I connected it to both large red and black wires for Mad Bypass.
Long winded, but what length should the fusible link be?
All the connections were a crimp/solder deal that I thought were all good, but there are several of them and am going to minimize what I can while I'm upside down lol.
Thanks
Steve

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I would not make it too long. "Let's say" max of maybe 3"