OK. I think it’s easier to explain that way.
Maybe because you used the word “cosine” and then asked whether the slider-produced curves should be exact or only similar.
This is a software fade so it is easy for advanced users to analyse what it produces and ask questions.

As you can see from the screen shots, a “cosine” curve (or to be more precise, a 1/4 cycle cosine curve) is quite similar to a logarithmic curve.
Which is part of the problem (if it is one) - what’s the difference? If you don’t know the answer in the first place, it is not as easy to Google and find out as you might think.
So to repeat my question “Is +51 log or cosine?” (if we don’t change)?
Thanks for the plug-in with the Cross Fade aka Square Root, Cosine and Log fades. I only tried fade out on a 10 seconds tone, but I find a lot of difference between Cosine and Log after the mid point. Cosine sounds smoother. Log seems to maintain the loudness much further towards the end. Inverting one and playing both seems to confirm that. Between about 6s - 8s the playback continues to become louder, then stays at a steady loudness, rather than get quieter.
Which is the “better” of the two for the “preset” - log or cosine? The log possibly sounds too much like the square root, so cosine might be more interesting from that point of view. That might suggest going for “exact” curves rather than “similar” for the slider, which thus gives a log EQ Power at +50, yes?
Do I guess that you still prefer “similar”, because the exponential curves otherwise have too much near-silence? Is this the more serious problem than having two quite different EQ Power presets?
I care more about having separate logarithmic and EQ Power (log or cosine) presets - even if you still think such a logarithmic (non EQ) curve would only be educational (I can think of a few cases where it would suit quite well).
Gale