If you have X manufacturing points being funneled into a project, then you get a modifier of min([BonusMax], [Value defined within the Bonus tags]*ceiling(X/[ManufacturingPerBonus]) from the project, where ceiling(X) is the function which returns the smallest integer Y such that Y >= X.
For example, if you funnel 103 manufacturing points into an Economic Project, you would add min(1, 0.05 * ceiling(103/10) = 0.55 to the colony's income multiplier.
At any rate, that was how it worked the last time I checked.