I would suggest replacing x with a

[beastr45]

for the user I think that its hard to catch on to the system in place because symbol x suggests a variable not another digit. The user likely will create complex rules replacing x with a number from base 10. X shpuld be replaced with A because its intuitive and follows sytems like base 16 which number a thru f.

[mwageringel]

Thanks for the suggestion. X can also be understood as Roman numeral, which I assume more people are familiar with than with hexadecimals. Interpreting X as a variable is also not far-fetched, as the first level conveys the idea that X not only represents 10, but also -1.