Pathfinder 1E Converting damage die by size?

I'm doing a new campaign, it's going to be set in an alternate WW2 Earth, and I have a question. Is there a program or chart to convert damage die by their size? There's going to be a lot of third party weapons as well as ones made for the campaign, and I'm wondering if there's a program where I can put the die numbers into and change the size to determine what the equivalent die would be? Particularly useful because there is a player playing a child in this party, and he would use small sized weapons.

The general rule is 1.5 times the max result for the dice for each step rounded down if there is no dice that fits.

D4-->d6-->would be d9 but there is no D9 so it goes to D8-->2D6-->3D6

The only oddity is that they seem to use 2D6 instead of a D12 for a max result of 12.