Forster type "rifled slugs" can be safely fired through ANY choke.
the slanted "ribs" on the slug, the ones that make it look like it was rifled, are NOT there to make the slug spin, they are there to allow the slug to compress, and pass through any choke (including FULL) without problems. Slugs don't spin (unless fired through a rifled barrel), they maintain stability because the nose is heavier than the back end, like a dart.
Conventional wisdom says slugs are most accurate fired through cylinder bores (no choke at all) but each gun is an individual, and might be the exception. Only test firing will tell the actual situation.
Sabot type slugs (Brenneke) of course, have no issues with chokes, as the slug is subcaliber anyway, and the sabot material (usually plastic) easily compresses and passes through tight chokes without issue.
Measuring the bore constriction and getting a number, only tells you what the maker intended the choke to be. Only shooting (at a pattern board) will tell you what the gun and THAT ammunition actually delivers.