First time eating any kind of THC food: Need Advice

I'm about to eat a THC food for the first time and just need some advice from experts on how much I should eat. Just bought a smore (just one, about a thick brownie cut size) has all the normal ingredients for a smore but with .25G of Cannabis butter, now I'm not sure if thats alot but just wanna make sure I dont over do it. Should I eat the whole thing or just half? Any ideas, help, well appreciated

Also Ive attached a pic so you guys can see what it looks like.

I wish I could get edibles but I would start with half if you never tried it before Ive had a caramellow 250mgs knocked me on my butt..You asked a really good question..:D