For a start up is there a down side for a smaller owner to have an anti-dilution clause protecting the individuals downside (eg 'not to fall below 10%', for ex). If they're ok, what's the highest % ownership a company should consider that won't negatively impact investors?
Are we talking about anti-dilution clauses for founders or investors?
In general, founders don't have anti-dilution clauses when there are venture rounds involved. Sure, there can be exceptions to the rule - but having such clauses are typically deal killers.
Of course, early stage investors always want some sort of downside protection, but there are times that their approaches become deal killers as well. Do a google on "Full ratchet anti-dilution" to learn about the perils of such a deal.
If they are investing in your company through a convertible note, then the downside protection is a natural part of the process. And yes, for the following round, that person will get the amount that they agreed to and signed at, regardless of what valuation you raise money at. However, to have some form of permanent downside protection seems unusual and unnecessary to me, and I'm not sure I've heard of such a thing. If they don't wanna get diluted down past that point, they need to contribute further.