Is there a spreadsheet that can help show how equity dilution occurs over a few rounds?

The amount of dilution will vary depending on company and stage. Everything can be negotiated (e.g. Sequoia was fine w/ <10% of Google when they initially invested b/c of how big the opportunity was). But in general, your institutional VCs for the Series A will want 25% (and can be negotiated down to 20%), and Series B will want 20% (and can settle for 15%). Your seed raise -- for your initial $1.5m -- it's not uncommon to dilute by 20%. But you should play w/ the spreadsheet and figure out how much dilution is comfortable given your equity share / # cofounders / etc.