Between 13.622 inches and 14.0 inches should be your sweet spot. My photogrammetry assessment of Gold 2 puts the length of T-bar 'past the Sat V engine cans' at 6% longer than what amount of T-bar is 'on the engine cans' itself, but other folks seem to think it's a dead 50/50 ratio. Your research and investigations may yield different results. Me personally, I tend to think "longer looks better" on Y-Wings, so long as you don't overshoot the overall ILM Y-Wing tip-to-tail length of 27.95 inches long. Even this is tricky, because the official length of the model is 71 cm according to canon books, but according to others who have measured the prop, some are as long as 28.52 inches long. Galactic Resin's version is 68.6cm, while the Nice-N Models version is 69.85cm. So, in truth, there is a whole lot of interpretation required to come to the right number.
The other rule worth following is making sure the Y-Wing is Fibonacci-ratio'ed all the way around, as I demonstrated years back. I suspect this is why the Fine Molds 1/72 Y-Wing is more aesthetically pleasing than the Bandai 1/72 Y-Wing, even though technically the Bandai version is more accurate, based on digital scans of the original, etc. When you just "scale up" or "scale down" from originals, you lose something if you forget the overall significance of proper proportionality of the parts to the whole. This is a long-winded way of saying that "if it feels right" or "if it looks right to your eye" then it probably is right, because it really will look good or look "off" in some way that's scratching the itch of the underlying proportionality.