Made a working roman calculator tablet with just a few rewrites, and the UX happen automatically by detecting that rules can occur when two symbols are in contact. I used a line for the intermediate values because..

In Roman calculation, stones are placed on lines representing units, tens, hundreds and thousands. The spaces between the lines are used to represent intermediate values that is 5, 50, 500. So it is never necessary to use more than 5 stones on a line or more than 2 stones in a space. This is because 5 stones on a line can be replaced by one stone in the space above and 2 stones in a space can be replaced by one stone on the line above.

XXXVIIII + XVIII = LVII