You don’t need a specific calculator, Just keep in mind that for parallel values of resistors the equivalent resistance will always be lower than the lowest value of any one resistor in the parallel combination. The easiest way to calculate the needed value of a parallel resistor combination is to convert all values of resistance into conductance. The value of any resistance in mho is equal to 1/R where R is resistance in ohms. As an example let’s say you have an 81 ohm resistor and you want to parallel that with some value to get 50 ohms. 1/50=0.02 – 1/81=0.012345679=0.007654321 mho of additional conductance required =1/0.007654321 ohms =130.6451613 ohms. Nearest standard value = 130 ohm 5%. If you want to find the parallel resistance of 2 or more resistance, simply convert each resistance to conductance, add them all together then convert back to ohms by taking the reciprocal of the total conductance, i.e. R total = 1/ 1/R1+1/R2+1/R3 etc. Example the parallel combination of a 100 ohm, 150 ohm, & 180 ohm resistors in parallel = 1/100 = 0.01 + 1/150 =0.006666667 + 1/180 = 0.005555556 =0.022222222 mho = 1/0.022222222 = 45 ohm. I hope this helps