Put the vacuum gauge back in the toolbox; you cannot use it to tune the idle. I stopped hooking it up for tuning, years ago.
At idle, your engine doesn't do anything but idle and doesn't much care what the vacuum is. Your engine only starts to care after stall speed has been reached. Between idle and stall, pretty much anything goes. But the biggest deal is getting rid of a tip-in sag or hesitation, cuz as you know, that sucks.
A high idle vacuum number means NOTHING, except that the throttle is closed. If it is too-far closed, you will have a tip-in hesitation.It cannot be otherwise, because the transfers are nearly shut off. and your Mixture screws out 2.75 turns is pointing straight to that. Close them up, a turn and give it transfer fuel instead. And keep taking timing out, and adding transfer fuel, until the sag goes away.
If you just cannot get it, you may have to crank up the metering rod adjuster, and level the bar, then start over.
A high idle vacuum does NOT indicate anything about engine efficiency. In fact, at idle, your engine is about as inefficient as it gets.
Want proof?
Put down your timing light. Leave the V-can hooked up and just pull the timing in until the rpm stops climbing. Now read the timing. That is the number required to get the maximum pressure delivered to the crankshaft, at just the right time, for maximum transfer of energy at whatever rpm the engine achieved. What did you get? 25*? 30*? 35*?
What is 13* compared to that number? Say you got 30*, then 13/30=43% of what it wants. What would 8* be? 8/30=27%. At idle, the engine doesn't care.
Your carb was designed to work at the factory settings. You cannot stray too far from them without encountering problems. A tip in sag or hesitation is instant proof of being too far off the factory settings.
Here's a shortcut;
set the timing to 5*, and the mixture screws to 2.5 turns, and the rpm to 600 in gear. Now, how's that tip-in sag? Remember, this is not testing the accelerator pump. So gently open the throttle as if you were gonna idle away from a stop. I cannot imagine a tip-in sag at this setting. But if it is there, it wants more fuel or less air. So you have to be confident that you have no vacuum leaks, that your secondaries are fully closed, and that your PCV is working right. If you are, then the next thing to check (Assuming the fuel is fresh) is the wet fuel level.