To be completely honest, I don't really view things from that frame. I think more about 1) how much does this company need to execute on their plans? and 2) what would a fair valuation be given their current progress and market comps? If that's 10% dilution then that's fine. If that's 25-30% dilution that's also fine -- but more dilution would be scary. Usually it's 20-25% dilution -- e.g. a fair pre-valuation might be $6m, and the company wants to raise $1.5m or $2m for 18 months to do X, Y, and Z.
If the valuation feels unreasonably high then I'll try to negotiate, or I'll just pass. If the valuation is low and results in too much dilution, then I'll try to figure out with the founder if they can make progress with less money/dilution, or I'll just pass -- because overdiluting sucks for the founder and will also eventually suck for me as an investor by capping the upside.
I think this framing works for the founder, too: if you need $X for your near term plans, and you're getting a fair valuation that doesn't overdilute you, then that's good. If the valuation is unfairly low, try to find other options. If it's unreasonably high, then that's okay if you are good with cash management -- but be mindful that the higher your current valuation, the higher investors' expectations will be for your next round.
