According to Yakima Chief:
Sterling is an aroma cultivar, a diploid seedling made in 1990 with a 21522 female plant and a 21361 male plant. Its parentage is 1/2 Saazer, 1/4 Cascade, 1/8 64035M (unknown German aroma X open pollination),1/16 Brewers Gold, 1/32 Early Green, and 1/32 unknown.
Used for its aromatic properties, similar to Saaz.
I don't think you'd be far off subbing Saaz. From what I hear, Sterling was widely substituted for Saaz during the recent shortage.
You can add varieties to the ProMash database. In the main screen, click the brown 'Hops' button. Select any variety by clicking on the variety once to highlight the entry. Click the 'Clone' button. Tell ProMash yes, you really want to clone the entry.
The cloned entry will be highlighted on the list. Click 'Edit'. Rename the entry - in this case, 'Sterling'. Update the average AA percentages. You can change the other stuff, too, but for homebrew purposes changing the values for Humulene, Carophyllene and stuff isn't really useful. You just want an entry for the variety with AA% so you can use it in your recipes. When finished edited, click 'OK'. Then click 'Save', then 'OK'. Congratuations! Your hops database is now updated.
Isn't ProMash fun?