I forge an item using Substat charm: 50% chance of Speed substat.

The item randomly happen to be boots.

Question: does speed charm reduce a chance of getting Speed as a main stat because it's impossible to get speed substat if that happens? 

Roll boots, then roll 1/2 if a charm works, then if it worked, speed main is removed from options.

Another implementation is rolling main stat first, and if it's the same as boosted substat, the charm does nothing. 

I can think of more ways how it can be implemented, this charm decreases a chance of getting boots in one of them.

Anyone knows or have thoughts about how it works?