Highway RPMS ?

Yeah, I thought that might be confusing..... lol.
You cannot ADD timing to what the distributor is commanding.
You can only retard the timing, it and then restore it.
How you use this device is to install the device with the knob in the fully retarded position, start your engine, and reset your Power timing at the distributor, back to whatever your engine likes.
Now, you can take OUT retard with the knob, up to 12* in the case of this
device, by dialing in "advance" which is actually just anti-retarding/restoring it..
My timing curve is pretty much all bugged out. I have a manual trans. At 700rpm, the slowest I can drive is 5.12mph . The only ways to drive slower is ;
to ride the brake until the engine stalls, or
to ride the clutch until I kill the clutch, or
to retard the timing so it will idle slower.
The idle-timing is 14*, and so I set the knob on mine, which has a range of 15*, so that I can retard the Idle-Timing up to 9degrees, which, in gear and pulling the idle speed might go down to 550/500. At 500 the roadspeed is down to 3.66mph, which is walking speed.
But cruising on the hiway at 65=2140, the cruise timing is only 46*. The engine likes high 50s. I can give it another 5 on the dial-back, total 51* which is as good as it gets for local commuting.
For long distance cruising, I reset the knob and Power-timing to get me a max of UP TO 46+15=61*, but I lose the 9* Idle-retard feature.