Buying a put to hedge a stock position

The IRC Sec. 1(h)(11) discusses dividends qualified for capital gain tax rate.

Specifically, dividends are excluded (IRC Sec. 1(h)(11)(B)(iii)(II)):

Such term shall not include any dividend on any share of stock—

...

(II) to the extent that the taxpayer is under an obligation (whether pursuant to a short sale or otherwise) to make related payments with respect to positions in substantially similar or related property.

So if out of the 400 shares you have such an obligation for 300 shares - then the dividends on 300 shares are excluded, and the remaining 100 are not.

So yes, you have it right.