I don't understand your math! Explain please 
4.5X*.5 - .5X = 1.75X = 21/16" = 1.3125" = 3.33cm
4.5X*.5 = half of space bar
.5X = stabilizer to edge of key.
Subtract stabilizer to edge from half of space = center to stabilizer
Also, since we subtracted the outer edge with the .5X, we can ignore the fact that each key is slightly smaller than the .75" spacing.
rest of the stuff is just conversion (I did it in imperial before noticing you're from Brazil.)