We’re talking about a balloon that we want to have rise to the stratosphere in 2-3 hours. If the balloon rises at 4 meters/second, or 7, instead of 5, it’s really not the end of the world. Just pick a size, calculate (rather trivially) how fast it’s going to rise, and then change the amount up or down.
If it really must be exactly 5 meters/second (or whatever), the computation is still at about the level of a slightly-above-average 8th grader.
A 20% margin would reduce space-access costs immensely.
We use exotic alloys, explosive fuels and precision machining because we need to reduce weight to the structural limits. If we hadn't, space access would be cheaper and safer.
Sadly, Earth has just enough gravity to ruin our day.
