Its a good combination, easiest and most exact way is to get a free GPS app on a mobile phone.
From experience, the insurance company is not looking for an exact figure, they just want to know if its non planing, planing, quick (50kts +) or Flippin Quick (65kts +). I think yours will do around 20kts + or -2kts, based on a 3.8 Futura FR doing 26kts with the same engine.
__________________
|