Question: efficient operation on sets

I have 2 questions here

1st I want to extend this set Ug3 := {1, 19, 23, 29} by repeatively subtracting 6 fron its elements. first pass should give

Ug3 := (Ug3-~ 6) union Ug3  Ug3 := {-5, 1, 13, 17, 19, 23, 29}

Now I dont want to add any elenments less than 6 so -5 need to go

Second pass produces

Ug3 := (Ug3-~ 6) union Ug3                Ug3 := {-11, -5, 1, 7, 11, 13, 17, 19, 23, 29}

Obiviously no elements less than 6 to be added on second pass either.

At this point there is no further elements that could be added to the set.

I am looking for an efficient method. I can use For loops and if then checks. The sets grow in length quickly. Repetition lengths are 6, 30 210, 2310,... I use sets because the numbers get ordered automaticially. Are sets efficient or should I use some form of table?