Round number but only to a percentage change

I have many unit costs. Some of them are large $18,911.25 and others are small $0.19. I want to round the numbers so that they indicate less precision. So the 18,911.25 would become 18,900 or maybe 19,000 and the 0.19 would become 0.2. One way I think about it is the rule I am after would be something like "round the number if the change results in less than 5%"

Can someone help me figure out how to make this rule?

Thank you!

Originally Posted by MrShorty

One easy way I have found to round a number to 3 significant digits:

I answered the question of what he wants, and although the OP did not say how many significant digits he wanted, I suspect that your solution is really what he needs .

However, it would help to know what he is using these numbers for, where they came from, and why he wants to round them, to be able to give the best possible answer.

You guys are very helpful. Thank you.

Since you asked, here is the background. I am preparing a cost estimate for construction. The values are compounded for inflation based on the future year of construction. This results in my unit prices having very precise values, when for this stage of design I should have fewer significant digits. I suggested keeping the round function from changing the original value -+5% so that the rounding does not have a large effect on the overall project cost. Once I get this working, it might be that -+2% (or some other value) is better. I tried both of your formulas and they seem to work pretty well. I have to play around and see which is better in the end. The Value / Text formula is pretty creative! Thanks again. If you have other thoughts or refinements based on this info, please share!

Personally I would round the bottom line number rather than rounding each constituent number, to minimize cumulative rounding error. Also, rounding by percentage is a bit unconventional, although I don't think there is really anything wrong with it. I'd be interested to see how this ends up.