What exactly mean X bit depth?
Bit depth is typically applied to a fixed point representation. 16-bit means the number can go from -2^15 to 2^15-1 so from -32768 to +32767. For 8 bits that would be -128 to +127.

If you normalize to a maximum of 1, your least significant bit (LSB) value is simply the inverse, 1/32678 = 3.0518e-05 for 16 bit or 1/128 = 0.0078125 for 8 bit

Floating point works completely differently. A 32-bit floating point number has a 24 bit mantissa and an 8 bit exponent. So it's basically a 24-bit integer number combined with an 8 bit scale factor that gets "optimized" and recalculated on every floating point operation. See https://en.wikipedia.org/wiki/Floating-point_arithmetic