Re: Serious Divers...
Re' Algorithms are notoriously different between different manufacturers,
Not so much anymore. Shearwater and OSTC use the same algorithms and will and give the same results. You can also run something like v-planner that uses the same algorithms and get your dive profile to write out on slates.
Suunto however do use their own closed-source algorithm which they don't publish details on and you can't use things like v-planner to match them so you're then in that very scenario you talk about, relying on a single point of failure and maybe if you remembered, their own software with no checks and balances.
In terms of setting the gradient factors this is where getting trained by someone like Mark Powell is a benefit. You actually get taught what they mean, how to set them and how/when/why you might want to change them.
Also for any technical diver, I'd say his book Deco for Divers should be a pre-requisite before you even start training.
Ultimately it's a fact that all decompression tables/computers/algorithms are best guesses to an extent. You can dive a profile that should be "safe" and still suffer DCI.