Electronic – 555 timer monostable time constants

If you have reduced Rt,Ct and the pulse is still too long, consider that the trigger input signal may also be too long. (this circuit is designed to stretch an input pulse, not shorten it!) In the circuit you linked to, that is controlled by C1 and R4 (though their instructions suggest it is controlled by C1 and R3)

Reduce C1 to 10nf and try again. Reducing C1 too far will make the triggering unreliable.

In the astable configuration, the low time is given by 0.693×R2×C and the high time is given by 0.693×(R1+R2)×C.

The factor of 0.693 is the scale factor (relative to the RC time constant) for the time that the capacitor takes to charge or discharge exponentially between 1/3 Vcc and 2/3 Vcc. The precise value is -ln(1/2) = 0.693147...

If you add the two half periods together, you get 0.693×(R1+2R2)×C for the total period.

If you take 1/period to get frequency, you can write it as 1/(0.693×(R1+2R2)×C), or you can move the constant to the top and write it as 1.442695/((R1+2R2)×C).