I'm sorry if I misinterpreted the situation, but have you even tried my suggestion yet?
In the ones column, the pattern 1,2,3,4,5,6,7,8,9,0 repeats 50 times, and the 6 occurs once, so 50*1. The tens columns goes from 0,0,0,0,0,0,0,0,0,1,1,1,1,1,1,1,1,1,1,2,2,2, etc. and repeats 5 times. 6 occurs 10 times in that range (60-69), so 5*10. Total is 50+50 or 100. There are no 6s in the hundreds column. But as you can see, from 501 to 999 there will be 6s, in fact 200 of them. Low numbers like 3 are reversed 200 in the 001-500 range, 100 in the 501-999 range. So you'll need to buy 2 sets of 001-500 to make one set of 501-999.
Yes, I could make a formula to do this, but a little analysis is quicker. Final recommendation: find another manufacturer. Seems like a lot of cutting.