C++ :: Linear Buffer Coordinate - Printing Onto Screen Coordinate

I have a linear buffer coordinate, i am to print onto a screen coordinate.

Say maxy = 20, maxx = 10, bufferx = 20, y, x,
Buffer coordinates to screen coordinates:
Algorithm: So for each maxx'th of bufferx, y will +1 and x will reset.
Mathematical: y = int bufferx / maxx,
const double averageIncr = int (maxx+1/maxx) - double (maxx+1/maxx);
So the amount of averageIncr in (int bufferx / maxx - float bufferx / maxx)
is the screen x coordinate. Then
(int bufferx / maxx - float bufferx / maxx) / averageIncr = x

Screen coordinates to buffer coordinates:
bufferx, y = 21, x = 10, maxy = 20, maxx = 10,

y * x would be the case if bufferx <= max screen coordinate.
There is no max bufferx.

How to find the transition from screen coordinates to buffer coordinates? Also, if the algorithm within a for loop, or that the mathematical way would be slower or not? I rarely touch floating point stuff.