Zero Products Torque Chart gives a somewhat typical treatment of "general torque values" for a variety of fairly common bolt types. As with quite a few such sources, a "formula" is given, but my personal advice would be that you not rely on this formula, or most other similar ones.

The discussion there about variations due to "lubricant" condition is valid, but note that the "formula" does not include any adjustment for the thread pitch – although the table gives different torque recommendations for UNC and UNF threaded bolts¹. If you actually used the formula as it's described, you'd get the same torque for both. This is the clue that the advice given is "partly based on magic" (i.e. things not revealed) which doesn't always work reliably for "nuts and bolts" work – unless you're the magician, know the "hidden details," and do some additional "adjustments" to the result.

¹ The unexplained correction also is in the "wrong direction" if you believe the "explanation" since he shows a higher torque for the fine thread than for the coarse one. (6 ftLb for 1/4-20 and 7 ftLb for 1/4-28). The lower pitch of the fine thread usually will develop the same bolt tension at a lower torque than for a coarse thread of the same diameter. The "error" in torque values is because he's actually used the thread root diameter to calculate his "bolt cross section area" without explanation, so we'll be forgiving and say it's not an error, just a "difference due to sloppiness."

Note also that in several places in the table the x/y bolt diameters appear as x1y, indicating the author didn't have a proof reader. (114-20 is 1/4-20, 112-13 is 1/2-13 and 112-20 is 1/2-20)

A minor quibble – to be a torque the value must be INCH-POUNDS (InLb or LbIn doesn't matter) or FOOT-POUNDS (FtLb or LbFt). "Pounds" alone doesn't cut it, although most of the shade tree mechanics probably will (usually) assume the "foot" value. Many US service manuals for more recent models do give the spark plug torque – for the "gasketless tapered-seat plugs – in INCH-POUNDS, and if you use the number with a FtLb torque wrench you almost certainly will strip the threads.

For metricized work, torques are usually Newton-Meters (Nm or mN) or Dyne-Centimeters. You may see kg-m specs, but those are technically incorrect and indicate an "uninformed" source. "Uninformed" is common and shouldn't be too disturbing.

You are unlikely to see dyne-cm values in auto maintenance/rebuild, since one dyne-cm is approximately equal to "100 flea fart angstroms" in actual value (i.e. – it's not very much torque).

Both intake and exhaust manifold bolts typically will use the 80% of yield torques. (The table at the link says 75% which is "in the range of common practice.") About the only common engine bolts that used the "full yield" torque values were older engine head-bolts, and sometimes for crank/rod bearing caps. Usually, even in very old engines, the bearing cap bolts used the lower torque values with a "bend over" retaining washer or other "anti-backout" device.

On "more modern" engines even the head bolts generally use the lower values with a "liquid gasket" or other zero-compression method that eliminates the need to bend the bolts. "Head gaskets" almost don't exist in recent engines, at least in forms like the old-style "compress-until-the-cracks-are-gone" kind.

John