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:
  • how much BTC I can spend in total
  • how much BTC I am willing to pay per coin
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.