changing water pump outlet connection from 1.25-inch to 1-inch, how to calculation flow and pressure
First off, the largest residential submersible pumps (5HP +/-) will not typically have 25 GPM flow at a total system head of 800' or so, so I think you might be a bit misinformed as to the capabilities of your pump.  A 5 HP pump I found showed about 4GPM +/- at 800' of head.  At that flow 1" pipe will be no problem.

Second, pump aside, 100L/min will flow way too fast through 1" pipe, so assume you will not be getting any where near that even if you could.

A typical way to identify flow from a pump through a system is to calculate an equivalent head, then reference the pump data to find the flow at that head (PSI instead of ft/m).

That is


  actual head + head from pipe losses (typically PSI per 100' at given flow) + head from fitting &amp; valve losses + target output pressure.


Say you had a bushing and fittings that added 2 PSI and 700'+100' of 1" pipe that adds 3.5 PSI per 100' @ 15 GPM + target output pressure of 50 PSI , you would have an equivalent head of about 875'.  Then you could reference the expected output on the pump chart at 875' of head and select a pump based on the total system head pressure.

Although again, to get the flows you are talking at the head you are talking would be a three phase higher HP (for residential) pump which would be rare.

Fluid dynamics is actually rather complex to predict.  This is why manufacturers use test and measure methods and then let you extrapolate on the data based on your total calculated head.