all that matters is the + you gain. that + is added to your proficiency in order to lvl it up.

so you can compound w/e yu want but only if it goes higher than the item you compounded will you gain any % so if you have a rank 4 and it becomes a rank 6 then you gain%. but if it fails to a rank 2 or 3 you gaain nothing.

thats why its best to use rank 1 items cause it will alway rank up with lil to no chnace of fail. get it now?

i tend to use barley, coal ore, copper ores or steam buns. because i tend to get +3 or +4 or higher with them.

tosses out what you don't need. keep or give away to newbies or keep for manufacturing. get more or just recycle back to rank 1 and start over again.