Release vang and ease mainsheet until the sail luffs. Pull outhaul on hard, then strap the flattening reef tight to the boom. Tension vang and the boom will set about one foot higher since you've taken about that much distance out of the leech. So flattening reef is either full on or off.
Agree with Bill that most tuning guides have the lower shrouds tensioned less than the uppers in light to medium air (for a bit of leeward sag) and converge to lowers/uppers tensioned the same in higher breeze. Earlier this year we did an 18 NM point-to-point race in 2.5 hours with steady breeze of 19 knots and gusts to 28. 41 on uppers and lowers which is about as tight as I can get on our turnbuckles and off the charts on the meter, but probably translates to 2400 lbs. It may be tougher for you to adjust the headstay with roller furling but in that breeze I would also go +15-20 turns from max. Goes without saying #3 jib.
What I was describing in the previous post was too much mast bend, where the luff curve is pulled so far forward that the main is flat. Could also be that the jib was back winding the main.
