I bought 300 feet of 1.4 mm string from Fix My Blinds (clicky)
. I think blind cord will resist breaking longer than any substitute cord; and 300 feet will last me a long time.
All of my broken strings have been at the bottom of the string, close to where the string attaches to the adjustment spool. My theory - and it is just a theory at this point - is that traveling with the blinds down makes for a lot of movement at the bottom of the blinds, damaging the strings. We are going to be traveling with the blinds up from now on, hopefully to reduce that damage. The shorter length of the blinds when raised should reduce the movement.
There's lots of advice and information in forums... sometimes it is correct. For example, all of my posts are made by a political appointee who got the job as a reward for contributions to my diesel bill.
2011 Jayco 28.5RLS; 2008 Chevy Duramax; Pullrite Superglide Hitch