Well, I haven't looked at your spreadsheet (bad me), but after spending more time than I should have on trigonometry and less on geometry, I've pretty much proved that if you ignore drag and assume that the aircraft is providing no aerodynamic side-force (i.e., that line tension is purely due to resisting the centripetal effect), then the line tension is the sum of the mass times speed squared divided by radius always, minus the mass times acceleration due to gravity times the sine of the angle above ground. This is regardless of the radius of the turn that you're executing (although realistically, the tighter the turn the more likely it is that the aircraft will roll and some of the lift will show up as line tension).
Less clear from the analysis that I've done so far, but pretty obvious once you realize that the only thing that lift can do for you when your wings are straight is to accelerate you along the surface of the sphere defined by the lines, is that the lift is the sum of a component that's straight "up" (or "down") on the aircraft and is proportional to speed^2 * radius of turn, plus the cosine of the weight due to gravity times the angle above ground.
This bears out my own observations, namely that on those occasions where the airplane is trimmed to not roll badly, the line tension seems to be primarily a function of aircraft speed and altitude: if I don't let it go too slow, it maintains line tension just fine everywhere, albeit lighter in the overheads; if the engine run is sick or if I'm really slamming it around & making it slow down, then the line tension suffers. And finally, to prove the importance of making sure that the assumptions are valid, if I don't have the roll trim correct for whatever reason, then the line tension will suffer on those maneuvers where the plane rolls in. Note that if you're trying to see this yourself you have to be on the ball: the line tension is a function of the square of the speed, and we're not always used to watching for the speed of the plane.