Product market fit is achieved by finding the successful intersection of product iteration, competition/market and go-to-market strategy. Finding product market fit (PMF), however, is hard when these three factors confound problem solving in the search for PMF.
Fortunately, competition tends to be roughly constant over the period in which a startup is solving for PMF. To control between product iteration and GTM, go-to-market can be broken into five sub-steps in any of which product changes are small enough not to confound. This allows GTM tactics and strategy to be tested and proven or disproven.
The five steps are first sale, founder sales, first sales person, sales leadership, scaling sales - each a distinct stage that can be tested and measured. There are metrics abound to measure sales performance, but many - including funnel conversion metrics, LTV and CAC - are fuzzy and imprecise in the early stages of a startup. What matters is whether a software business is adding adequate net new revenue per cash burned as measured by monthly increase in MRR per monthly net cash burned. Cash efficiency should go up at each successive go-to-market step.