All numeric entries in horizon support basic two-operand math as well as some other goodies.
They look like these:
Both point and comma are recognized as a decimal separator regardless of locale settings. By default all numbers are treated as millimeters. Suffixing a number with an i or in will treat it as inches. Use mi or mil to convert from thous.
Additionally, two-operand infix math is supported, so you can to this:
Addition: 1+2
Subtraction: 1-2
Multiplication: 3*2
Division: 3/2
Average \frac{ a+b }{2}: 3|2
Adding mm and inch: 1in+2mm