Help me in attenuator calculation please!

I need to solve a little math of Attenuator but I am little unfamiliar with dB vs. watt. There are 3 resistors:-
R1= RF to ground
R2= Series in RF line
R3= again RF to ground

Here are two devices-
1) 1 watt FM Transmitter (12 Volt)
2) 5 watt RF amplifier which needs 500mW as an input

Problem is to connect between them. I decided to make an attenuator myself but cannot calculate the values of these resistors to drop Tx's 1 watt to 500mW (to feed amp). help me please!

Is the nominal output impedance of the 1W driver 50Ω? If not, what?

Is the nominal input impedance of the 5W final 50Ω? If not, what?

The design of the attenuator requires the impedances...

The Pout of the attenuator is 0.5W, the Pin is 1W, so the attenuation is 0.5, which is -3db.

If you know the impedances, here is an online calc. This makes a three-resistor PI circuit.

You could also use a three-resistor T network. (There are calcs on line for that, too)

Sorry, cannot handle this Pi Attenuator Calculator, so please help me. Tx and amp (which i have) is not professionally bought, it is experimental. So I think input and output impedances are between 50 to 75 ohm. So, show me an example of 60 ohm (average). Show me example like this way which will guide me to use this calculator too.

Filled in calc values with rounded resistors values

I think asking a very basic question with you (Coz of my lack of knowledge):- how to measure impedance of particular stage? Few ways? Please