Do what we all do, chuck a bit more on evenly over the field using the money saved from detailed mapping/testing.
Kind of missing the point. Generally yield maps (for me anyway), show nutrients are not the limiting factor to yield, even when they are deemed as sub optimal. Why throw that 'saving' you mention on more fertiliser when the better return might be drainage/moling, improving organic matter in the soil for improved workability or moisture retentiveness. Etc, etc?