Tax being calculated incorrectly

I have the sales tax set at 8.375%. The cart added $1.70 in tax to an order for $20.50 when the tax should have been $1.72 (technically 1.716875, give or take a few digits). What can I do to fix this? If I make up an item with a total price of $20.50, the item page correctly adds $1.7169 as the sales tax. Where is the error coming from?

Without all of your settings it is difficult to tell- but it appears that you do not have enough digits after the decimal to round properly.

How many digits are you supposed to have? 8.375 is the tax amount I input 8.3750 because that is how the cart altered it when I added 8.375 as the amount. How many more zeroes would affect rounding up?

Still having a problem, even after rounding the tax rate up. I am using 8.38% now and it still charges too little for the tax amount. It can't be a rounding up problem because the tax is off by more than one point each time.

Again, the item setup page shows the correct tax amount for the item. When you go to checkout, the tax is too low for the entire order. I have only one tax rate installed and only one tax class installed, so it's not picking the wrong tax to add, it is just adding it wrong.

What can I do? This is not a test cart, this is a cart that is receiving orders at this time.

Also, this is a cart owner question as opposed to a tech question on the install side. How does a cart owner alter the order amount without adding one of those add-ons in the contribution section?

Here is an example.
Last order received. Taxable total is $30.80
Sales tax at 8.38% should be $2.58
Tax added by cart is $2.56

I just read another thread on this where Dr Byte explains how the cart compiles the total tax by adding the item total, taxing it and rounding up, then taxing the shipping total rounding that up, then adding the two together. There is still a problem.

The item total comes to $27.80
Sales tax on the items comes to $2.33 ($2.32964, to be technical)
Shipping is $3.00
Sales tax on the shipping (yes, NY state charges sales tax on shipping, be glad you don't live here) comes to $0.25 ($0.2514, again to be technical)
Adding both tax amounts together gives you $2.58
The total tax added is still $2.56

It is still less than the actual tax amount based on the tax rate of 8.375%