Proximity of nitrogen fixers

Just moved onto 2.5 acres and am planning my approach. I have a good handle on guilds, but with my annual veg garden I am wondering: how close would a nitrogen fixer need to be to have a positive impact?

I’d like to try lead plant or baptisia minor to fix nitrogen for the annuals, but I’m not sure how close I’d need to get them. If I plant in fairly standard rows 30″ wide with 18″ rows between, would planting some in every other row have the impact I”m looking for? Or would I need to plant them every 2nd or 3rd plant in every row?

I don’t feel like I’m explaining this super well, but hopefully you catch my meaning.