I'd start much lower to be on the safe side.
With very-likely-real 5-IAI that was going around, I got strong negative effects from like 10-20mg, felt very empty, depressed and dysphoric. Certainly not typical effects, but i'd have hated to jump in with 150mg!
Another reason for caution is that the majority of 5-IAI making the rounds is NOT 5-IAI. 5-IAI looked amazing on paper. It was hyped to hell, and every RC manufacturer started trying to make it. Then, the pilot batches came in, and people started taking it, and realized that it wasn't nearly as good as it looked on paper. That's why every vendor stopped talking about 5-IAI last year. I'm told that the stuff that I have is good, but not amazing, at really high doses (250-400mg has been quoted to me

), but I don't care to find out, considering what 10-20 did to me.
So, what did the vendors do now that the next big thing was a dud? They started selling whatever they had lying around and calling it 5-IAI! Usually meph or random stimulants. Some of this was done without the vendor's knowledge, with suppliers upstream selling fake 5-IAI. The majority of 5-IAI is something-that-is-not-5-IAI.
Honestly, 5-IAI isn't a drug that's worth exploring, afaiac - the real thing isn't very good, and the fake ones are god-knows-what.