It will reduce the fueling about 7% overall. It's meant to either provide a mild adjustment without using a programmer, or supplement a maxxed-out setup that needs headroom in the air/fuel charts.
Carefully scan your air-fuel ratio, make small/smart adjustments, and it should work fine. I typically adjust my fueling (in tune) by the percent difference (%diff) that the wideband O2 is away from my commanded air-fuel ratio.
Example: I scan a full throttle run, and see WideBand O2 (actual) = 12.3:1... Commanded (tune) = 11.5:1... so ~7% leaner (higher) than I want it to be; ((WB-Cmd)/Cmd) x100%. I would either change that range of my tune +7%, or do the same by setting my AFC to 107%.
It's a bit more difficult without a wideband, but you can carefully monitor knock and regular O2 readings to be in the ballpark.
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