You can't figure out the stress on a word ending in -ative from the corresponding word ending in -ation. That's because, as far as I can tell, all words ending in -ation are stressed on the second-to-last syllable. (This is true of the word declaration; the stress on the first syllable is secondary stress, which often occurs two syllables away from the main stress of a word.)
If it's two syllables, the stress is on the first syllable.
If it's three syllables, the stress is probably on the first syllable. This may be different from where the stress is in the related verb.
- reláte, rélative
- negáte, négative
- sedáte, sédative
- abláte, áblative
- narráte/nárrate, nárrative
I know of only one three-syllable word with the stress on "-at-":
If it's four syllables or more, the primary stress is on the third-to-last syllable or earlier. (However, some polysyllabic words that end in "-ative" and have primary stress earlier than the third-to-last syllable may optionally have secondary stress on, or at least an unreduced /eɪ/ vowel in, the second-to-last syllable; e.g. "ímitative" may be pronounced as either /ˈɪmɪtətɪv/ or /ˈɪmɪteɪtɪv/.)
To get an idea of where an -ative word of four or more syllables is stressed, you can look at the related verb, if it exists. According to English Pronunciation in Use, by Martin Hewings,
In words ending -ative, stress is usually on the same syllable as in
the root word.
However, there are some exceptions to this rule that are just stressed on the third-to-last syllable instead, like "interrogative" (which is stressed on the "o", even though "interrogate" is stressed on the "e").
with the stress on the syllable before "-at-":
- accúse, accúsative
- decláre, declárative
- consérve, consérvative
- represént, represéntative
- provóke, provócative (note: the vowel has a different sound in these two words)
- fórm, fórmative
with the stress two syllables before "-at-"
- spéculate, spéculative
- admínister, admínistrative (with possible secondary stress on the second-to-last syllable)
- coóperate, coóperative
And there are also some words that don't have a corresponding verb (or at least not one that's commonly used):