If someone is doing these kinds of calculations client side, then they are pretty much begging someone to take advantage of them like this. After all, it is code running on my computer that you've given me control of by doing critical decisions client side. I've just decided to make that critical decision myself using the same code that has been provided to me.
Don't ever do critical calculations like this client side. It's just dumber than dumb.
So is exceeding the speed limit. However, catching speeders is much easier to do than proving someone intentionally manipulated the data client side and that it wasn't a bug somewhere else in the shittily designed system.
Don't ever do critical calculations like this client side. It's just dumber than dumb.