Flawless Spell Casting

"Any and all spells are, at their core, the extremely shortened version of a ritual.

As such all spells consist of an incantation in various forms. The second component of a ritual and also a spell is some form of symbols, in a ritual, it is circles, penta-, hexa-, hepta-, and octagrams or some exotic figures. Sometimes runes, other symbols and so forth. In a spell, it is mostly movement, of the body, the hands, feet, head etc. alone or in combination with each other.

The last component of a ritual is a material component. A spell does not have such, it has been extracted from the spells matrix. With that, we have the core difference between a spell and a ritual.

Now, with the rise of the wand, staff or other focus as a primary tool for doing magic, spellcasting has become even more popular in the last centuries. It has indeed come to a point where rituals are no longer a focus-point of magical education and training.

The key to understanding spellcasting is to understand the spells basic structure and based on that the control over the incantation and the correct and precise execution of the movements required.
And here it is where the focus comes to power. The use of a focus allowed the spellcaster some leeway, some imprecision in his movements. The better the focus is adjusted, matched to the caster the stronger this effect.

One has to factor in that the use of various different foci is not only a matter of comfort, availability, custom or fashion.
Each type of focus has its own advantages and weaknesses. The staff may be a better conductor of massive amounts of energy but lack some precision in its movement. A ring will complement and empower hand gestures but not help at all with body posing or foot movement. And a wand may help with precision but will burn out when channelling too much power.
And so each focus can and should be considered only helping in casting."

From the chapter on spellcasting and foci "Fontibus Ex Quibus magicae ritualia" by Magistra Fahimja sunya Riftah sâla Laila ay Achmad´Ra recently published from Institution et Academia Scientifica

As the Magistra stated the power and quality of the spell comes from the precision in casting it. This means that you can expect better results the better you are at the casting.
For TDE/DSA you could use the skill level for the spell, and adjust that level for correct focus etc. by +-1 for the various things. Then divide the resulting skill level by 6 and round down, and add that number to the quality level. Alternatively, additionally or in older editions do the same but decrease the cost respectively add to damage duration etc.

For D&D5e I suggest a proficiency for the spell from 1-3 and adding that to the casting roll if you use such a rule. It is however totally useless if you do not use any system to define success or failure in spellcasting.