rubber track pitch line question

[KaosDwarf]

I'm trying to replace a rim driven track system with a sprocket and having trouble getting my teeth to match track profile. Where do i measure track pitch line at? Rims were approximately 16 inches in diameter and don't want to change gear ratio too much or have to modify my track rollers significantly. My diameter is off so after rotating the sprocket and track aren't lined up. My track is 72 mm pitch and thought an 18 tooth sprocket would be close to the right size. Thanks for any help or suggestions.