The viscosity test
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What the number is
Saybolt universal seconds is a timed efflux measurement: how long a sample takes to flow through a standard orifice at a stated temperature. Under 100 SUS at 100F is thin. Over it is not.
Why it is in a packaging regulation
Because thin liquids are the ones a child can swallow quickly and aspirate into the lungs. The hazard the rule is addressing is a property of how the liquid moves, which is why the threshold is a flow measurement rather than a concentration.
The same number decides your closure type
A liquid thin enough to fall under the regulation is also thin enough to run out of a spouted closure, to drip from a brush and to pass straight through a disc top. The regulation and the dispensing decision are reading the same physical fact.
So measure it, do not estimate it
A formula near the line is exactly the formula a producer will guess about, and it is the one where guessing is most expensive. The measurement is routine and it settles two questions at once.
Thickening is a real option
Where a formula sits just under the threshold and the function does not require it to, thickening above 100 SUS removes the requirement altogether. That is a formulation decision with a packaging payoff, and it is only available if somebody checked.
Temperature is part of the test
At 100F. A product measured at room temperature is being measured at the wrong point, and most things of this kind thin appreciably over that range.
And the exclusions are narrow
Spray devices, and products in which the liquid cannot flow freely. A paint marker qualifies. A bottle with a brush cap does not, because the liquid in the bottle flows perfectly freely once the cap is off.