As mxyzplk mentioned, yes, your formula for calculating saves is correct, and no, it does not directly factor in your character or class level, but it does count the spell level of the spell. That is a class-level-dependent value, and it matters.
Note that your highest-level spells should be mostly keeping up with enemy saves. This will fall short somewhat with monstrous enemies with good save progressions and a high HD:CR ratio, but with enemies who gain roughly one HD for every 1 level you gain, the increase in their base save by level should match your increase in max spell level (and thus in max DC) for their good saves. Ability scores are even more favorable: both sides gain at the same rate per point gained, but you're likely to grow your key stat more than they grow their save stat.
On their poorest save (which is the only one that should matter: enemies have weaknesses for a reason, target them), your situation is even more favorable on both counts: your spell level grows half again as fast as their base save, and most creatures, thanks to design following themes, tend to have low abilities tied to low progressions, while your saves are based on the ability that should be the main focus of your investment. Even with relatively high HD:CR ratios, their poorest save is unlikely to be growing faster than your DCs except with outlier monsters, and if you invest heavily in save-boosting feats and items, you may even keep ahead of the outliers.
Stick to high-level slots for things that require saves; fill any slot more than one spell level below your highest based on the assumption that every target will always succeed every save they call for. Make your key ability modifier a priority. And above all, match your targets to your spells. Learn what kinds of enemies are vulnerable to what kinds of spells. Your goal should be to make it from level 1 to 20 without ever once casting anything that calls for the type of save your target applies their highest save modifier to.