Math
You can calculate the End frame within blender. Enter the fps and multiply it with the time in seconds, resulting in the number of frames.
End Frame
Since blender starts counting at zero, to get to 48 frames, we should subtract one frame, i.e., the End frame should be 47, or?
This puzzles me also, but not that much 
if you want to make a looping animation.
Your last animation key frame (a copy of the first one) must be +1 from the loop.
First key-frame at 1
Middle key-frame at 51
Last key-frame at 101
But you render from 1…100 !
In the latest version of Blender, we see frame 1 as first.
In the older ones, frame 0.
I think the intention was to start at 0, but render from frame 1
Maybe these thoughts will help you.
