Electric mining drill mines 30 ore per minute (wiki) i.e. 0.5 ore/sec
Electric furnace processes 36 ore/min <=> 0.6 ore/sec
Unloading a chest on a belt with a
- regular inserter 0.86 item/sec
- fast insterter 2.1 item/sec
Several relations derived from these:
If feeding electric furnaces straight from electric mining drills, you can get away with 1:1, however, 5:6 should be better
If feeding them from fast inserters (eg. train station), the relation is 3.5 furnaces per, or 7 furnaces per 2 fast inserters.