Ender 3 w/400XL, 4.2.7, CR Touch

I installed the CR Touch & downloaded & installed FW from Creality. The problem is the FW doesn’t know about 400x400 build surface. I’ve sent a request to Creality to create FW that works with my 400x400 build surface. I attempted a work-around by signing on Martin where I have an account & creating FW w/400x400x500 build column & BL touch. Compiled & downloaded it & install it. I ran the Auto home, Move axis, move Z. The screen said Z axis was +10mm. Attempted to move the Z axis down to a negative value, but would not go below 0.00. At that point the nozzle was ~15 mm above the build surface. Any Ideas, suggestions, help.

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