What is the equation

Okay @SamTheFox

i haven't tried an adaptive program, but you can add a wait (0) seconds block and set the wait to the time to the periapsis divided by the mass of the craft, minus the thrust of the engine(s) in the stage, but i have no idea so you'll have to experiment with it by yourself

That seems to be showing how to do the burn, I just want to know how to take the TWR and other info to calculate how many seconds away from apoapsis to start burning @SamTheFox

you will meet the same requirements i said earlier

Perhaps I was a little unclear, @SamTheFox I was asking how I can make vizzy calculate when to start the burn, not how to make it start

at this point, you don't really need an equation for this, you just need wait blocks

on start
wait until (orbit Apoapsis > planet (name of Planet) Atmosphere height)
lock heading on Prograde
set throttle to 1
wait until (orbit Periapsis > planet (name of Planet) Atmosphere height)
set throttle to 0

but if you want more complex mechanics, then you will need equations and wait blocks