If you mean your overall score, that really depends on what sort of scores you've already posted. But using a victory method other than military will indeed result in a lower score; the modifiers are based on how hard/easy the different methods are perceived to be.

OTOH though, if you check out the AltMeta instead of the official metaverse, there you can submit whatever victory type you feel like doing and compete with those posting similar games, rather than being forced into the gigantic-suicidal-military mold to compete on the official meta. So long as you're in an AltMeta listed empire (or make your own empire of one and request to be listed), you just post games normally, and the AltMeta gathers your results from the official meta a couple times a day.

The overall score has been figured out by others, though I don't recall it right offhand. You are right though, scores (as they contribute to the overall score) are aged a certain amount each month down to a minimum value (65%?). Then those aged scores are summed up and divided by an amount based on the total number of games submitted. It's this function of total games posted that can cause your score to go down if you post a small one at the same time as the divisor increments.

The altmeta is pretty simple. Scores age 1/6 of their value each month (to 0 after 6 months), and the altscore is just a sum of the aged scores. That way stagnation is prevented--you have to keep playing to keep your rank--and there's no penalty for playing many games.