[maxmcd]

There's a few ways I've gone about doing this in the past:

- Find out if they've ever done something similar in the past and see if you can get performance numbers on that post.

- Ask around and see if you can get some good Facebook click data numbers for the type of post you expect to get. These vary greatly depending on the brand but you can usually use engagement (comments and likes on individual posts) to see if different user groups are going to perform similarly.

- Do some basic back of the envelope math. Look at how many likes they have, likes per post. What percentage are going to click on your link?

Your best bet is usually the first one. Similar campaigns on the same page are going to offer you the best insight. Although, I've found that social media tends to fail to meet my expectations time and time again in situations like this. Depending on the company, you might not push that far past 1k/day.

Either way, good luck.