The standard approach to calculate the box price values in a Point and Figure Chart is to start the smallest value up. This is the default. But you can select From Largest on the Parameter screen should you wish to switch to using the top-down approach.
For example, From Largest will take the highest data value plus a margin and then calculate the box values down the chart by subtracting the boxSize (or multiplying by 1 - boxSize% for logarithmic charts). From Smallest starts with the lowest data value less a margin and calculates the box values by adding the boxSize (or multiplying by 1 + boxSize% for log charts).
As with any P&F chart, changing the price values in the boxes can have an impact on the detailed shape of the chart and the emergence of buy and sell indicators. Changing the direction in which the box values are calculated (especially for logarithmic charts) will change the values shown in the boxes (by a small amount). Hence changing the box calculation direction From Largest to From Smallest can result in small changes to the P&F chart.
Both approaches are valid but From Smallest is the method used in most charting software (e.g. StockCharts). Chose one approach for a stock and stick with it.