Leftover XP is optional | 0 is equivalent to leaving field empty
Possible calculations (clicking on a list item will empty the fields that are to be calculated -> clicking the last one will empty everything except current level):

• you enter current level, goal level -> you get required resources to achieve goal
• you enter screws, cogs, metals, expedition tokens, current level -> you get max achievable goal level
• you enter three resources (of screws, cogs, metals, expedition tokens), current level -> you get max achievable goal level and required amount of 4th resource