Magnetic field distribution within the fixed coil on a wet horizontal bench.

Hello guys I want your opinion on longitudinal magnetization using coil shot. As per what I have read the magnetic field is theoretically zero in the coil center and increases to a maximum value at the inside surface of the coil. I’m confused to why some standards have a formula for current calculation for parts placed in center of coil. So is the field zero in the center or is it more uniform in the center ?
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