'm having issues on how to properly extrude a sketch on a curved surface.

tldr; i need to extrude a circle with a specific height on a curved surface

The diameter of the circle that aligns the pipe inlets/outlets is 400mm, in which 2 of them have to be 170mm but when i boss-extrude them i can either Blind and specify the 170mm or Up to surface but then i don't have the value so i would need to know if the plane in which this sketch is placed is a the right distance right? I've tried to reverse the direction but then (while using Blind option)  i'd need to know the height of where that 400mm is touching the upper curved body of the tank so i could extrude upwards? 

There's probably a way more easier way to get this done cleaner which is why i'm writing this, hope someone can help.