Standard 'base' tuning for a set of SU carbs is to screw the jet adjuster up until it (gently! as goes for all adjusters on any carb, obv.) it tops out on the carb body, then three full turns out.

Unless the carbs are wildly unsuited for the motor (totally wrong needles, improper fuel bowl level setting, etc), this should be good enough to make it run half decently. At least as good as it needs to be for cam break in.

The recommendation mostly has to do with the rather casual nature of the lubrication the cam lobes get in a B18/20 motor. There's no direct lubrication. There is some drip down from the lifters, but they aren't directly lubed either - they just get some of the oil that comes down from the pressure fed rocker shaft/rockers. Some of that oil goes down some dedicated drain holes, some drains on top of the lifters to lube them, a very small amount probably leaks past the lifters.

The main way they get lube, then, is by crankcase windage. The spinning crank with it's pressure fed bearings constantly spewing oil out (especially the rod bearings) makes quite an oily fog inside the block. The cam lobes are positioned fairly decently to get splattered by the rods. (Note that the lobes are not able to take advantage of any pre-start up lubing by spinning the oil pump)

It's still a somewhat indirect way to lube something, and that's a pretty stressed friction surface (especially if you have double valve springs and a big lumpy cam). One of the reasons lifters can be problematic in a OHV redblock motor. If you started the motor and let it idle calmly for a while, it's possible that the lobes wouldn't get adequately lubed, and gall slightly before they've developed that finely honed working relationship with the lifter.

You should use assembly lube on the lifters as well, sometimes the new cam will come with some special grease, if not you should use something similar - not a thick oil that will drain off before the thing gets completed and installed and started.

Also, modern oils have sharply reduced levels of zinc in them (more precisely, whatever chemical compound ZDDP actually is). Zinc is very good for sliding steel surfaces like a cam lobe/lifter. This is partly because modern engines have fewer such interfaces (more roller style lifters/cam followers), but mostly because catalytic converters don't like zinc from whatever oil gets into the exhaust. This obviously isn't an issue for (most) pushrod redblock cars. So either search out an oil with a higher ZDDP content, or add some ZDDP directly. Lucas makes a 'break in oil' which is a BUNCH of ZDDP in a bottle, on a brand new motor I'll put a whole bottle in for the first 1000 miles or so, then switch to something like 1/3rd a bottle in every oil change. I started doing this after having some repeated lifter issues in the PV's motor (big cams, double valve springs, plenty of revs used frequently).
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'63 PV544 rat rod, '93 Classic #1141 245 (now w/16V turbo)