Resistor calculator to get close to a final value?
Once you have got accustomed to the existing resistor values E24/E48 and to certain extremes even E96 and E192, there is no real need for such application/website.
First of all, with a bit of experience, you can guesstimate the value of two resistors in parallel, bu knowing for example that 2 resistors of the same value in parallel result to a combined half the value of each.

In general when you select the resistors you start with the E24 series, and then, depending on the required tolerance of the circuit, you adjust to the series with more values. In most cases you will find that there is no need to put them in parallel or series, because the value you need already exists, or the ones available are within the tolerance of the circuit.

For example when it comes to setting the resistors for the feedback of an adjustable voltage regulator, you start with an arbitrary value for one, and then you calculate the second resistor. If this one is nowhere near an existing value, you change the first resistor and recalculate again the second one, and so on, until you find a combination that works for you. I usually enter the formula in excel, and then play with the values, until I get what I want. Remember that as long at the output of a regulator is stable, the circuit will work even if you supply it with a 1% higher/lower voltage.