I think that when changing "price" or "total" in "BUY TWINS" that the "amount" should change instead of the price or total fields because the most important values that I already know are:

To me it also feels more natural that the amount acts as the result from the values of these two other fields.

If we do change the "amount" I believe that the price field should change because the most important thing that I already know (out of "price per coin" and "total amount to spend") is how much I want to/can spend in total because that value is closest how I'm truly limited when it comes to how much I'm buying.

I assume that some people might want to set the amount to buy and the price per coin when for example creating a masternode or buying something for specific price using TWINS but I believe that the values that directly relates to my funds is the most important ones and therefore should act as the values producing the result.