Pros and cons to each approach. I tend to recommend that it in the early days companies build up by employee, and then look at the metrics coming from the other direction as well to make sure everything makes sense (e.g. if you're forecasting that sales will triple, can you accomplish that with the hiring you've projected).

Main reasons for this are:

- It enforces discipline. It's easy to make hand-wavy assumptions like forecasting costs as a % of sales and calling it a day

- Small changes to the hiring plan can have a drastic impact on a startup's finances, including the timing of those hires

- If you tether all your expenses to sales, you can't really explore the downside, because you'll always be showing consistent profit margins

To your point though, I agree that it's important to look at expenses both bottom up and top down.