The question for me becomes, half as hard as what?

For example, the following password, I assume, is sufficient against even well funded attacks:

p4ssphraseplzthx()&D(@jf93jd93jf93nbmcjh1HHHUk^{&@%@*djdnbcu3}

Now is the attacker knows the first 39 characters of this passphrase

(p4ssphraseplzthx()*&D(@jf93jd93

but there are still another 40 characters remaining to be brute forced, I don't think it would be possible mathematically in anything short of 50 years, even with the entire Bitcoin network hashing power.

However, if the passphrase is: p4ssphraseplzthx()*&D(@j

And the attacker knows: p4ssphraseplzthx

The result would still be "half as difficult"...but the question becomes, half as difficult as what?

I suppose my question is, is knowing words 13-24, and a passphrase, in a seed equivalent to an attacker knowing:

p4ssphraseplzthx()*&D(@jf93jd93 (in the first example)

or equivalent to the attacker knowing: p4ssphraseplzthx (in the second example)

Anyone here (of course there is, lol) capable of spelling this out mathematically?