active antenna attenuator, what value of pot to use?

Hello, I have built this antenna preamplifier and it works nice
http://www.sentex.ca/~mec1995/circ/Activ8/activ8.html

I would like to insert a pot between the antenna and C1 in a voltade divider configuration, to achieve input attenuation when needed. I have tried a 10K pot and it works nice, but is that the right value I should place there?

An input potentiometer isn't a good idea. Assuming that the input transistor isn't yet overloaded, you can use a potentiometer in place of R3, connecting C3 to the wiper.

Why an input potentiometer is not a good idea? Maybe because of impedance problems?
If going as you desctibe, strong out of band (and adjacent) signals will overload the fet and rise many unwanted signals.

Saying so, I have not noticed any difference in the signal strength with the potentiometer in place (turned to max sensitivity) and when leaving this out

The potentiometer will add noise and possibly load the antenna, depending on it's impedance. To find a better value than the original 10k, the antenna impedance should be known, too.

Because you most likely don't know it, keep the 10k potentiometer or add a local/DX attenuator switch in front of the amplifier.