Constants are set per environment and cannot be changed from the runtime. You can however use them in calcuations (with @module.constantname). If you start a calculation and you want to retain the initial value it is best to start the Microflow with a create variable and store the initial value in this variable (and don't change it during the calculation).
Note: at the end of the Microflow this variable is not available anymore unless you pass it as a parameter to another Microflow or store it in an object.
With kind regards,
You could set a default value on the attribute, so that it is set when the object is created. You can then restrict user rights so that the value cannot be written to only read.