Example: A warlock wants to limit its Eldritch Blast to one beam instead of using all four as his high character level allows him. In this case the warlock wants to weaken a creature, not to kill / blast away it for later interrogation. If a spell caster has trained a spell, he should have a kind of control about that.
In the books is said something like this only by chosing spell slots to use. I have no ideas of I missed something. And spells like Eldritch Blast have a power by character level, not spell caster level or spell level. Complicate thing.
Answer: You have to houserule and suggest WotC to publish it in official texts 😉
Must I burn the whole storage by using Burning Hands or can I limit range and power? Essential question if wood and straw are surrounding me. SUGGESTION: If a spell caster wants to limit the effects of a spell he knows, he has to use a bonus action or action for more concentration. By this the spell caster must succeed a spell casting ability check hardened by a modifier: -2 for 50% of range or beams or creatures or hit points etc.; -4 for 25% or 75% of such an effect; -6 for 10% or 90%. If the spell caster wants to limit several effects of his spell, the modifiers stack for the ability check. If the spell caster fails the spell works full power, if successful casted at all.
But remind, some spells have such a limiting inclusive. Example: Misty Step let you teleport in range 30 feet to a seen point the spell caster’s choice.
SUGGESTION 2: Like 1st suggestion but instead of an extra ability check use modifiers halved on the ability check for success without using a bonus action or action.
Both suggestions can be used in the game, by choice of the spell caster which method he wants to use for this moment.
What are your suggestions?