Then let's take the opposite point of view... How does a unit's double strike differ from a single strike of another unit, but with twice the damage?.. Essentially, we can make a simple substitution. Instead of a Crusader, let's take a Manticore: the average damage of both units within a single round is identical (all other things being equal). So, on what basis do we apply a coefficient of 1.75 to the first unit? Wouldn't it be simpler to include the doubled damage (not the attack) in the calculation scheme and forget about the boosting coefficient altogether? This would be more accurate...

Hmm...
Maybe I'm not expressing my thoughts clearly enough, but when considering a "one-on-one" fight, that's how it will be. But in this case, we lose an objective (real) in-game assessment of the unit's strength.
And when considering a "group" versus "group" fight, we find that a full-fledged double strike is only possible if the "attacked" unit doesn't retaliate. :)
In 70% of cases, after the first strike, there is a response, the number of warriors in the stack decreases, and the second strike turns out to be weaker.
The question is, what coefficient should be used for the second strike...
The average damage dealt by a unit is equal to 1/4 of the unit's HP, therefore the stack will decrease by a quarter, which is directly proportional to the damage. Taking into account the expected percentage of response to the first strike, we get a value for the strength of the second strike in the range of 70-80% of the initial value.
There. :)

I would like to draw your attention to the fact that it is not the attack that needs to be adjusted, but the damage... Because the developers have incorporated a completely different calculation mechanism for the situation when the attack is higher than the opponent's defense and for the situation when it is lower... By modifying the attack, we will unjustifiably change the calculation mechanism, which will not actually happen in reality...
Added 7 minutes ago
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Too much, IMHO...

In my case, by "attack," I mean not the attack skill, but the unit's attacking (combat) coefficient. I would like to note that my entire theory of unit evaluation is based on the sum of the unit's attacking (power: the basis is damage and attack skill) and defensive (viability: the basis is defense skill and HP) scores. + boosting coefficients of abilities that relate to (affect) either defensive, or attacking, or general characteristics.

And yes, it's a bit too much; I suggest reducing it to 10%. Well, here it's just a guess. :)