price + tax rounding

[mlieqo]

Hello,

wondering if anybody has some experience with prices + tax calculation?
I've been dealing with a following problem:

I am supposed to display product prices in cart already with tax included, but
let's say a product price without tax:

quantity = 1
price_without_tax = 285.07
tax = 1.21    # 21%

so that means I should display product price with tax:

price_with_tax = price_without_tax * quantity * tax     # 344.93469999999996 
price_with_tax_rounded = Decimal(price_with_tax).quantize(Decimal('.01'), rounding=ROUND_UP)     # 344.94

So the price I display for product is 344.94. The problem is that if I do the same calculation for
two products:

quantity = 2
total = price_without_tax * quantity * tax     #  689.8693999999999

I would like to get to get to price = 689.88 which would make sense,
but rounding gets me to 689.87 which I guess can be confusing for customer. Also I need to add tax
on the "total price" and not to every product individually because, sometimes I need to do some
discount subtractions on total_price without tax.

Any tips?

Thanks in advance