Since you are already using 50' line, how do you like them?
That's a great length for the real light stuff. Personally, I would keep it under 75' in real light winds. I generally keep it under 85' in any wind
I fly in UL / SUL winds a lot and I usually fly #80 lb @ 60-70'. My most used UL set is 80# 58' with 6' leaders. I still use 50# if the wind is super light and stays that way, say under 2mph but mostly I just use the 80# in light wind no matter what. If it picks up or is fluctuating and you're flying a full sized kite like a Nirvana, the 50s can easily break.
Another thing, and why I always use leaders even in light wind, is that if you are tricking and wrapping the line around the kite, thin line can cut into the kite. I use leaders every time I fly.