I would DEFINITELY load it longer rather than shorter for pressure concerns.
If you are not running into the lands and grooves with it seated farther out, seat if further out or change bullet manufacturer. Are you loading the Remington 405? If yes, they have lousy cannelures. They are cost effective, but have that bad attribute.
Those cannelures are there for a reason.
Can you measure some actual 45-70's at the gun store? Usually, folks are pretty good about helping you out.

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