What am I looking at?

Example: Pokémon White 2, a 512 MB ROM. 146 MB with no dictionary, down to ~109 MB at the far end.

Each point is one dictionary size. Across the bottom is how much of the cart's 8 MB PSRAM the dictionary would occupy; up the side is the resulting .ndz file size. The amber point is the recommended balance.

It's a sliding scale

PSRAM is shared. Whatever the dictionary doesn't take becomes decompressed-block cache while you play.

More dictionary = smaller file on your SD card.
More cache = fewer SD reads and fewer repeat decompressions, so less battery drain and fewer load hitches.

"none" still compresses

Block compression and the per-block filters always apply. Choosing none just skips the shared dictionary, leaving the entire 8 MB as cache. It's the best-battery, most-responsive end of the scale, and the file is still far smaller than the raw ROM.

Where to land

The curve drops steeply, then flattens. Most of the savings arrive in the first couple of megabytes; past the knee you're trading a lot of cache for very little size. The amber point marks that knee, but it's your call which end you want.