Field Culture Upgrades?

So on the wikia, it says every 2.5 points of farming skill reduces the point cost to work on a field culture by one, it also says the labourer skill reduces field culture cost by 1 for every 2.5 points.

I have 20 labourer (20/2.5= 8 ) skill and 61 (60/2.5=24) farming skill, so reasonably it seems like I should be spending 68 stamina [100-(8+24)] per unit to upgrade the field culture, but it tells me it will take 80 stamina per unit.  So is the code wrong or the wiki wrong?  The only way I could see it working with the wiki being wrong is if its -1 per 5 farming skill, in which case [100-(8+12)=80]