groundwater hydraulic gradient confusion

Hey all I'm getting really hung up on what length (distance) to use in the hydraulic gradient equation. In the solution below, they use the "diagonal" distance between the wells. I would have just used 175 ft. The example from this link also used the "horizontal" distance https://www.quora.com/What-is-hydraulic-gradient

Which one is the correct value to use?!

I would say its negligible. In problems where slope or length is within the equation, mannings or friction loss (darcy/hazen), I would really look at the datum and the difference of head elevations . If the difference is considerable, to the point  the length changes substantially due to the large head difference, then I would consider it. In this case if they are giving you the angle then just use it. But I doubt your answers will be that close as to account for approximately 4 ft difference in length.