Thinking about buying this or a Thomson seat post for my caad9 build:

https://www.artscyclery.com/descpage-PROV7SP.html

Not sure what length to get. I've determined I'll need approximately 7 inches or about 178mm of seat post extension between the top of the seat post clamp and the seat rails.

The problem is I don't know what the typical minimum insertion length is for a typical aluminum seat post. If I get the 280mm length that leaves approximately 10cm of insertion into the frame. Is that enough for a 180 lbs guy like myself? Also, when they say 280mm I don't know if that's usable seat post or the entire length from the base to the top of the clamp.

Can anybody help me out here so I know which post to buy?

I believe the length stated is the entire length of the seatpost. The general rule is that you want the seatpost to go past the intersection of the seat tube and the top tube at a minimum, and most seatposts I have seen have a minimum insertion mark at least 3 or 4 inches from the bottom.

Well if that's the case then the 280mm should work. It's about 21 inches from the center of the BB to the top of the seat post clamp, and then I need another 7 inches from there to the seat rails. 7 inches is approximately 180mm, leaving 100mm or about 4 inches into the frame. That's a little over an inch below the top tube. So it should work, but I'm still wondering if I should play it safe and go with a 350mm post.

I could make this easy on myself and find out what the minimum insertion length is for both of the posts I'm looking at. I'll have to search Google images to see if I can find anything.

Chart from the Thomson site, covers the question well.

I just purchased a standard Thomson seat post and had to exchange it for setback post. Do some checking on this, again on their site. Most seat posts have some amount of setback, the standard Thomson has none and would not work for me. The setback one is sweet.

Get the longer Thomson (I believe the price is the same) and then cut it if necessary.

it looks like 250mm would be just at the limit! id probably go 3xxmm because you might get a thinner saddle one day.

Yup. I have the 31.6x287 post but I knew from my original seatpost exactly how much would be external and was sure it was enough.