So Hunter's Machete is a good item for jungling, I love it. Makes Attack Speed marks great. But if I'm playing a more tanky jungler like Nautilus, my options are Madred's or Spirit Stone. Spirit Stone builds into better items for Naut so that is the clear winner. The problem is that in order for Spirit stone to be better then Hunter's Machete I need to be doing 200+ damage every auto attack, which is very difficult early game. I shouldn't be paying gold to lose power, this is why Spirit Stone damage to monsters needs to be buffed. If you don't agree with me, I would like feed back on why you choose this as the design choice.
My suggestion: Bump it up to 20% for Spirit Stone, and leave its upgrade also at 20% you are paying for the stats on the Tier 3 items, not a better jungling time.
You aren't taking into account the extra 5% damage amp to your abilities (Passive, W, E) as well.
I know there's a math-y crossover point at which you need to pass in order for Spirit Stone to be higher damage in some cases due to the loss of the Rend passive. This isn't ideal, but we acknowledged this and accepted that players can opt to leave the Hunter's Machete in their inventory in order to go for other items until they feel comfortable that upgrading Spirit Stone would be a for-sure damage increase.
Not sure if this will make it into the next PBE build, but I changed both Spirit Stone and Madred's Razors to retain the Machete's Rend passive. The Madred's / Wriggle's passive proc is now renamed to Maim.
Wriggle's and the final Spirit Stone upgrades will still not retain the Rend passive.