A 50mm overlap on the 250mm board leaves you with 0.20m x 2.6m coverage, or 0.52m2 coverage per board. 

A 30mm overlap on the 100mm board leaves you with 0.07m x 2.4m coverage, or 0.168m2 coverage per board. 

Finally, measure the area in metres squared (m2) that you want to cover, add 20% for wastage, then divide by the coverage provide by your cladding piece after overlap. That is the number of pieces you need. So for example, a 40m2 area with 20% additional for wastage, is 48m2, and using the 250mm board and a 50mm overlap as in the example above, would require 48m2 divided by a coverage area of 0.52m2, or 92 pieces required.