Impedance matching resistors for digital output
The 390Ω || 91Ω will give you 73.8Ω looking into the TTL gate output from the load end which should be terminated in 75Ω. So this part had been set-up for matching the source to the tx-line and the 75Ω load. All you need to do is simultaneously ensure that the values of Rx || Ry = 75Ω, and that the voltage division generated by your Rx-Ry voltage divider is 0.5V into a 75Ω load to give you the whatever current you're after. The final Rx and Ry values should satisfy both conditions.
One equation will resolve to (Ry∙Rl)/(Ry∙Rl+Rx(Ry+Rl))∙V=0.5
the other will resolve to (Rx∙Ry)/(Rx+Ry)=Rl
Rl is your load impedance and V is your source voltage. You'll need to solve the simultaneous equations for Rx and Ry.