I use JB 500# and it feels super solid. With splices and not knots you don't lose any of the rating, so I feel safe with 500# as I stay on the ground
as well, and it is a lot thinner than the 800#. Pre-stretched properly it feels like a solid cable to the kite, there is no springiness at all.
Really in the end it's what you feel comfortable with personally. I weigh 140# and I'm using the lineset on a 4-line depower setup that transfers half
the load to the brake lines, which are also 500#. So I feel it's unlikely that any one line will ever experience even a shock load in excess of 200#,
giving me a huge safety factor.
If you are flying FB then of course you can use much thinner brake lines. At some point they become too mechanically thin, tangly and scary, I tried
100# Spectra lines once for an experiment and they are like cheese cutter wire, yikes! I would keep it above 200# just for handling reasons.
I'm building another 22m lineset with an eye to doubling up at some time for 50m to try the super-long lines thing myself. I would splice your lines
up for you but I feel I don't have enough experience yet to risk someone else's life or equipment. I think some of the online kite shops will make
custom length lines and I'm sure some of them will chime in soon. I don't know if any will splice provided line though? I know BHP tackle will splice
loops onto JB line but I wouldn't trust them to be accurate enough with the length for kiting.
Doing it yourself is not hard if you have good dexterity and quite a bit of patience. I like the locked brummel splice as it's rated for 95%+ and you
don't need to stitch or whip it, but you do need to do some tricky passes through the line. You can make them yourself for about half the price
quoted. To make 50m lines you need to know what you are doing and properly pre-stretch and measure, because small errors can add up fast.
Homebuilt: 1m NPW9b, 2.6m NPW21, 7m NPW21 UDS
HQ: 3.2m Crossfire, 5m + 7m Apex 3
6m Ozone Explore v2