That would probably produce the most accurate results.

On the other hand, there's almost certainly a spreadsheet for this kind of stuff with powder weight, ball weight, bore diameter etc. Most likely they were just taking the necessary info from something like this.

If not, they could also calculate it. It wouldn't be easy and would require a bit of reading but they should have the math and physics background for it. Cannons and the physics behind them are extremely well studied. I'm absolutely sure that there is no lack of material on the subject.

So long as you have the stats on the black powder and the dimensions of the ball and the cannon, you could also calculate 'the hard way'. Some university level thermodynamics and a bit of basic physics should get you there easy enough. It would only be an approximation since friction and other losses are probably not easy to calculate but it should at least give you a solid upper max speed. Either way, it's absolutely doable.