Off the top of my head:
I know only the EPCOS brand RM8-cores
The +30/-20% tolerances are worst-case - usually much better within-batch. I measure a couple of sets from a batch for reference, and 90% of the rest are usually within 5%. High-AL cores like T38 are even worse rated at +40/-30%, still most are 5% within-batch.
I found it is apparently important to keep the cores in the matching sets they're delivered in from factory. Maybe they do some core-half-matching at production/packaging?
Very important to "grind" the core halves onto each-other after assembling the inductor, while reading inductance. You'll see inductance rising to a point where no further grinding brings improvement, leave it there. The reading is NOT your final inductance value, it will drop a bit over the next days.
If some inductors come out too wrong, simply swap the cores and put the wrongies aside marked with their "real" AL-value for different use.
A simple yet effective calculator for most of your coiling needs: http://powermagnetics.co.uk/calculator.html - does all sorts of calculations between Turns, AL, and inductance.
Jakob E.
