I believe you meant to write that the stock concentration is 5 U/µL. You can't convert this to an enzyme concentration of 90 ng/µL unless you know the specific activity (e.g., in U/mg) of the stock solution. 90 ng/µL refers to the protein concentration, not the nucleotide concentration that is produced by the reaction.

The specific activity may be written on the label or in the product information sheet or certificate analysis for the particular lot of the product that you received. This information may be available on-line from NEB. Once you know the specific activity, the conversion is simple.

For example, suppose the specific activity is 1,000 U/mg.

You need 90 ng/µL = 90 x 10-6 mg/µL

90 x 10-6 mg/µL x 1,000 U/mg = 0.09 U/µL

Since the stock solution is 5 U/µL you must dilute by a factor of 5/0.09 = 55.6-fold into the reaction.

Or, you could do it this way, using the same example specific activity of 1000 U/mg:

(5 U/µL) / (1000 U/mg) = 0.005 mg/µL = 5 µg/µL = 5000 ng/µL

The required dilution factor is 5000/90 = 55.6

You could try to measure the protein concentration directly by using a protein assay, such as the Bradford or BCA assay. Or, you could test various concentrations in U/mL in your experiment to see what works best.