What you could do at this point is take 2by stock and notch it out 1/2 or 3/4 by 2 inches depending on how wide the "stringer" is next to the tank. Now the notched out area sits down on the existing stringer and extends up past deck, snap a chaulk line at desired height. Its kind of confusing but I hope this helps. Just one method of many. I'll try to post a pic of the concept but I think it may be tiny ( can't resize)
