I haven't tested the 143X with RL26 yet (did with H1000 and 7977 though with great results), but did with the 156EOL. Not the same of course but will add my info as a data point.
With the 156EOL, I hit max pressure at 55.8 with Hornady brass and .02 jump. Max safe load netted an average of 3016fps on a 3 (yeah I know, not 5) shot group. I started at 54 grains and worked up in .2 grain increments up to 56.2gr with a drop tube and sloooooow pour before running out of case capacity. Everything from 54.6 on up to 55.8 (max load I shot) resulted in cloverleafs. Will retest at 300-400 yards to see what holds together and further tune the load from there but, velocity, SD, and accuracy were all very promising.