Ah well i think i was fuzzy with my goals here.. I made the 30/120/54 gear change and threaded a ER32 collet that way.. Yes it works, but its a sloppy thread. Its accurate enought to take thread and putting force on it works..
But... (isnt there always a but).. when putting the same collet in the horizontal feed with different drawbar the slop becomes more apparent.. And i thought to myself.. well i could prob. redo this. But should i recalculate the thread to come closer to the ratio that was originally used. That 5/3 ratio.
But you kinda pointed out that my math was way off there.. =)
Converting the thread to a ratio by doing some excessive simplification doesnt really work out.. the 1.666mm thread, that can be read as a 1 and 2/3 of a milimeter, is a 5/3 ratio. But cant be calculated as i tried to.

And now when this is cleared up i need to revisit this damn buttress in a other fashion i guess to get the thread tighter and well better. =)
I do must say i get a better and better understanding of how to cut and calculate .. annoying thread pitches. ;)