vacuum gauge to solve idle issue

Leave the fast-idle screw out of it, lol; it is for setting the fast-idle speed when the choke is activated.
Yes the only way to adjust the transfer-slot exposure is with the curb-idle screw.
Very few carbs ever had an Idle Air Bypass. You have to engineer your own by introducing air below or thru the primary throttle valves. This is essentially a controlled vacuum leak. It is not usually required until the cam gets up to about 230/235*ish @.050; and even then it will be just a bit of air. By 240 tho, things are different; the 292/292/108 likes quite a bit of bypass air.
Some guys crack the secondaries, but if this is dry-air I haven't had much success with that.It seems the back cylinders go lean, and it's hard to work with that. And if you add fuel to that air, then of course the idle speed goes up. And then you have to either close the primaries, which upsets the sync; or you have to retard the timing.
So on street type cams, I don't usually crack secondaries.