What percentage ownership of a private company doing $50M+ in sales should a CMO get? The company has had 4 rounds of financing ($100M total) and is on IPO path. Realistic IPO valuation is $500M according to bankers.
I'd be surprised if a company like you described would be asking this question. The question to ask isn't "what percentage" but rather "how much value" to give.
Ultimately, rather than determining a percentage of ownership, you should be determining a dollar figure of value that you think the role is worth, then offer them that value as a combination of cash and shares.
You mention 4 rounds of financing... based on that financing, you should be able to determine a share price for the company. So, as an example, if you determine that the share price was $1/share, and you want to give the person $100k in cash and $100k in shares, then you'd be giving them 100k shares as part of their compensation package.
This is, however, an exceptionally rudimentary example. You'd need to balance a few other questions as well:
Best of luck!