If you get someone to film your landing, if you know your exact altitude for your first turn and then calculate your speed using hight and time?
Personally I used input of back risers as stop for the timing.
Or if you can decide exactly how long it takes to cover say 700 feet up higher and then calculate. Must be possible with Optima or Neptune? (performing 360s while timing)
This is what I did (the first option) but I was my own idea so maybe ask for some advice about this first.
(If it's a stupid idea I'm sure someone will point that out below soon....)