So, for the increase drop rate purchases, how much exactly does it increase by? I know someone previously asked percentage wise, but I'm thinking more in units. Would it just be by 1 unit?

For instance, I'm in the 3rd cave atm with 67% stone, 25% Copper, 17% tin. I believe it was stated that it rolls a number from 1-100, so in my case 1-17 is tin, 18-25 is copper, 26-67 is stone, and 68-100 is nothing. Is that correct?

If i bought an increase to stone drop rate, would it be 26-68 instead of 26-67 then? By that logic, wouldn't it also be completely not worth my time to buy a copper increase since 8 tin increases would make it impossible to get copper altogether?

Thanks in advance.