old mod 200mm new mod 310 new tip 0.16129032258065 does this sound about right?

I thought it would have been closer to the 0.1mm than that though.

cause the 200mm was @ 0.25

At the risk of causing more confusion...

In going from a 100mm diameter to a 200mm diameter you doubled the diameter.

If you multiply the diameter by 2 you change the collecting area by 2 x 2 = 4. So with 4 times the area each tip will be caused by 1/4 = 0.25 as much rain as with no modification.

If you increase the original 100mm to 310mm, you have increased the collecting area by 3.1 x 3.1 = 9.61. With 9.61 times the area each tip will be caused by 1/9.61 = 0.104 as much rain as with no modification.

Does that make sense?