There is an article about inclined manometer datum height at
http://books.google.com/books?id=zLwtngK3T1UC&pg=PA52&lpg=PA52&dq=manometer+datum+height&source=bl&ots=Ue8TuGDwNQ&sig=3cx6cTfh0dM9ke9UWC7uxW6OHrY&hl=en&ei=FywBTJePA4HGlQeGoK3yCQ&sa=X&oi=book_result&ct=result&resnum=6&ved=0CCUQ6AEwBQ#v=onepage&q=manometer%20datum%20height&f=false
It seems you need an area ratio for the two colums of fluid that the manometer uses. read the article and see how you can use it to predict datum height d.