Hi @squigglebit,
If you have a custom lifecycle stage for the trial, you could create a custom funnel report, as explained here: Create custom (legacy) funnel reports
Instead of selecting all lifecycle stages (All created contacts, Lead etc.), just select your “In trial” lifecycle stage and “Customer”. Then make sure to choose the “Show contacts that have gone through all stages” option. HubSpot should now show you, for the date range chosen under Filters (top left) how many contacts where in “In trial” and which % of those moved to “Customer”.
Best regards!