Going in the opposite direction it would appear, when replacing a 6mm rod with a decimal rod, I've always used the .230 For example, 6 thousandths is very tiny small amount of difference, so small that a single wrap of Scotch tape will make your rod too big for the ferrule. By comparison, it's somewhat harder (at least for me) to accurately sand off exactly 4 thousandths (maybe 5 for good measure) and I've always felt that removing the outer coating off a rod made it that much more likely to break or wear out. You could always coat a 230 rod in some fine powder
(no, actually baking soda comes to mind. What were you thinking?) which should take up the extra space. If you're worried about fine tuning, then you might want to replace broken spars in sets, so maybe both lower spreaders. Honestly, I wouldn't worry about it myself, except in the case of lower spreaders, where different flex might be bothersome. Most of my old English kites are repaired with decimal spars, and I've noticed no difference in performance at all. If internal reinforcement is desired, I'd just use what you can find. If you don't have a ready supply of small gauge carbon or fiberglass around, I've used bamboo with success . Just saturate it well in CA glue, then be really fast in shoving it in, and trim off the excess.