I have several 4 strokes you can't hear the top rpm and can't hear them drop a few hundred. I don't always use a tach but I can dial an engine in a lot better with one. Steve, you want the tank center line just a touch lower, about an 1/8 or 3/16 then the needle valve or spray bar if you go inverted. Not a big bunch. I use foam on my tank compartment so I can add and subtract it if needed to adjust the tank. If the fuel is siphoningit usually tells you there may be a tank location problem. Some engines don't care as much as others. There is no real fixed rule, it's just my starting point for any inverted engine that isn't pumped. Other people will have other ideas on the subject, I try not to etch anything into stone.