Salty: hard to quantify down to a simple number like that -- there are many factors at play that lead to it... its not JUST nutrient availability -- its a combination of factors, as Karin stated. Light quantity/quality, nutrient richness, flow rates, and a number of other factors can help encourage the growth. I don't know that anyone can say specifically "at this point, nuisance growth starts".... do remember, your readings are always free levels of nitrogenous wastes -- if the algae is growing, there is an immeasurable amount being consumed -- could be high or low depending on growth.

-Justin