Math.ceil depending on the number of decimal places

Hi,
I've got an issue and I hope you can help me to get this going.
Well, to start with, we all know what ceil does:

ceil(x) - Rounds up the value of "a" to the next integer. If the value is a already whole number, the return value will be the same as the passed value.
Code:

The example returns 13. resultval = Math.ceil(12.01)

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The issue:

In particular this is about a tax rate of 12.5% whereas totaldue_ is the total of a given shopping basket and ship_cost_ is a given figure, say 7.91:



Now, if I for example would need to only ceil decimal places (2) I'd do something like that.



That gives me the ceiled total
totaldue_ = totaldue_+ship_cost_+tax;

So far everything is fine and everything works as expected as long as the sum of tax_t = totaldue_*0.125; and/or tax_s = ship_cost_*0.125; has more that 2 decimal places, for example:



The following causes the problem for me:



Can you see the difference?

In the event that I get a number with less than 3 decimal places I don't want to ceil. Only in that particular case I rather need to floor or do nothing.

So I guess I'd need something to determine the number of decimal places against the result of that math. How could I realise that?

Cheers
Jan