u/PhilTheQuant

Look, I can already do that, sans medication.

From a metrology (measurement science) perspective I believe the real answer is none.

The SI system initially defined a lot of things relative to a number of physical standards, not least the standard kilogram. The original metre (possibly before it was SI) was defined relative to the Earth.

However, over time those units have been redefined in terms of a fundamental or observable aspects of the universe; the second is defined in terms of the number of periods of a Caesium oscillation. The metre is defined in terms of the second. The kilogram used to be defined by a physical block, and now is defined by the Planck constant.

By defining a unit in terms of counting or relative to other unit which are defined this way, you can avoid needing an absolute unit at all.

I'm not aware of any units in SI which could not be defined this way in the end, so as a result the answer would be none.

Can you give us a bit more detail on the hole vs the cross sections of the sticks? I take it they're not tight in there?

Bottom left: there must be a queen in one of the two left in the first column, so you can exclude the two squares adjacent to them.

In the same way, row 4 you have 3 squares remaining, so just as if it was one colour you can exclude the squares above and below the middle of the 3.

The rest proceeds from there.