One more thing which might help, Patjack.
I was chronographing several calibres today doing a series of Minimum to Maximum 'ladder' loads using new powders. Some of these weren't listed in any reloading manual so a bit of extrapolation or guesswork was needed from previous results in other cartridges.
Among them was the Sierra .243 100gr SP (ProHunter #1540) tested in my Tikka M595 - standard 22" (57cm) barrel.
I followed the min./max. given on the Hodgdon website using IMR 7828 ... which I use in heavier calibres, but haven't used before in .243 Win. This powder is listed in the Hornady #7 and #8 Handbooks but not in the Sierra #5 Manual or Speer #14.
Hornady say the absolute MAX is 45.0 grs of IMR 7828 at 2900 FPS ...... just as they say a MAX of 34.2 grs of IMR 4064 gets 2800 FPS....... which is under 1700 ft.lbs.
The higher 'Maximum' of 46.0 grs IMR 7828 listed at 3009 FPS from the Hodgdon website I decided to use (their 'highest velocity' load) only returned 2853 FPS, and 2874 FPS.
That's a higher maximum giving an even bigger shortfall. This load is just 'legal' and as I can't cram any more in the case without special measures it's a nice safe one.
It leads me to think that either the.243 is a marginal calibre, or more likely that none of the reloading sources including Hornady can be relied on without doing your own testing.
Even taken as a rough guide they're p*ss poor.
If you follow the link below you'll find yet another different load for your IMR 4064.
http://data.hodgdon.com/cartridge_load.asp