How to mathematically determine the frequency of a tone based on components being used
The formula is correct, but it's normally given as:

period = 0.693&times;(R1 + 2R2)&times;C

Since frequency = 1/period and 1.44 = 1/0.693, your formula is equivalent to this.

R1 is the variable resistor in the schematic, R2 is the fixed resistor, and the capacitor C is the one attached to R2. Units are in ohms and farads.

ADDITIONAL NOTES:

In case you're wondering where the 0.693 comes from, it's the natural log of 2, and this derives from how the 555 timer operates, driving the capacitor voltage between 1/3 Vcc and 2/3 Vcc.

Be aware that in the example circuit, you cannot allow the adjustable resistor to go all the way to zero ohms, as this would then tie the DIS (discharge) pin of the 555 directly to the power supply. The first time the chip tries to discharge the capacitor, it will short out the power supply.