$3-5.$ When measuring levels, if the distance between the two points to determine the height is far, a curve error due to the curvature of the earth and a refraction error in the atmosphere of the light beam occur. Answer the following questions about these errors.

$(1)$ What is the range of the horizontal distance for the combined error of curvature and refraction to be less than $5mm?($the radius of the earth $R=6,370km,,$ refractive coefficient $K=\frac{1}{7})$

$(2)$ When leveling is performed across a wide river or ocean, errors due to combined error of curvature and refraction can be reduced by averaging the values observed from both sides. This method of measurement is called reciprocal leveling. $P$ When the ground height of the point is $40\mathrm{.}68m,$ Find the ground level of the correct $Q$ point. $($However$,|APBQ,AQBP|$