Without remembering the math, a smaller gauge wire like 21awg would need more windings to be equivalent in magnetic strength, so I would expect the coils to be sorta similar. But there is a lot more to the formulas than that. Best thing is to do is replace with the correct coil. As for the "gives" you mentioned, remember the 2 windings are in series during the hold, so yes the coil could have weaker strength overall in both of its duties. Power stroke = heavy winding only (high current), hold = both windings, (Low current) for max power the eos switch should be gapped maybe 1/32" when flipper is in hold position.

The right flipper has the stronger coil. It has fewer turns of slightly larger diameter wire so it will draw more current leading to stronger magnetism.

I agree with Student Prince. The FL20-300 is definitely the stronger coil.
The 20-300 coil should have a resistance of about 1.1 ohms where the 21-375 should be approximately 1.5 ohms. The 20-300 will draw about 1/3 more current than the 21-375 and yield more power on the flipper stroke.

Had the same issue on my OXO recently. replaced the incorrect left 21-375 coil and it was a dramatic difference. 20-300 is much more powerful

It's complicated. You'll find formulas and calculators online that will usually show that the magnetic field strength
increases with:
- the number of wire turns or windings
- the amount of current passing through the wire
but decreases with:
- the length of the coil (which decreases the winding density)
- the distance between the windings and the plunger or core

Many online calculators simplify things and assume a single layer of wire windings but coils usually have many layers of windings, each a bit further from the core. Calculating the current can be problematic too as the current through a coil will be determined by the wire gauge (thickness) and the configuration of the coil winding.

These factors all combine in an elaborate and non linear way to determine the magnetic field or coil strength.

Consider the common practice of removing some windings to make a pop bumper coil stronger. Fewer windings should decrease the strength but in reality it increases the strength because you're removing the outer windings which are relatively weaker and because you're lowering the overall coil resistance which lets more current through. The effect of the extra current through the coil outweighs the effect of fewer coil windings to a point. If you were to continue removing windings however eventually your coil would become weaker. Imagine removing all but one winding. That single winding would quickly overheat and would have little if any magnetic strength.

Electricity and Magnetism is typically a 2nd semester college level physics course so it's no wonder that there isn't a simple and accurate formula for coil strength.

/Mark

We are dealing with AC here. More turns means higher inductance hence higher reactance, further reducing the current taken by the coil. The coil with fewer turns will be stronger.