Which modifier do you use when casting a spell that requires a spellcasting check from a spell scroll?

Question

The Dungeon Master's Guide has this to say about Spell Scrolls :

The level of the spell on the scroll determines the spell's saving throw DC and attack bonus, as shown in the Spell Scroll table.

• Cantrip - DC13 - +5
• 1st - DC13 - +5
• 2nd - DC13 - +5
• 3rd - DC15 - +7
• 4th - DC15 - +7
• 5th - DC17 - +9
• 6th - DC17 - +9
• 7th - DC18 - +10
• 8th - DC18 - +10
• 9th - DC19 - +11

But what about spells, like Telekinesis, Counterspell and Dispel Magic, that involve a spellcasting ability check instead of an attack roll or saving throw ? Which modifier do you use when casting a spell that requires a spellcasting check from a spell scroll : your own, or one that is extrapolated from the above list (for example, considering that the proficiency bonus of the usual 1st level caster is +2, then the spellcasting ability modifier related to a 1st level spell scroll would be +3) ?

Answer 1

Rules as Written

But what about spells, like Telekinesis?

Telekinesis reads:

Make an ability check with your spellcasting ability contested by the creature's Strength check.

The rules on Spell Scrolls reads:

The level of the spell on the scroll determines the spell’s saving throw DC and attack bonus

According to the written word, the ability check would be made with your spell casting ability, since the rules do not replace this use in the specific rules for spell scrolls.

Intent and Reasonable Rulings

It is impossible to gather if this was a mistake they didn't care enough to fix, or if this is the rules as they intended.

It would not be unreasonable for a DM to rule that you use any of the following:

• the spell attack modifier on the table
• the spell attack modifier from the table minus a constant value of 2 (the base proficiency bonus at level 1)
• the spell attack modifier from the table minus your proficiency bonus

As long as the ruling you make is applied consistently, all of those would be within bounded accuracy of 5e. And, given the relative rarity of scrolls in most campaigns, none of those would be game breaking.

Answer 2

To add to J.A Streich's answer:

The table from the DMG corresponds to the stat progression from the leveling tables.

If we assume that a caster starts with a 15¹ in their primary casting stat during character creation and then adds +1 or +2 from racial bonuses, they'll end up with +3 to that stat at level 1.

If we also assume they increase that stat by +2 each time they get an ability score improvement, they'll end up with a spellcasting bonus identical to the "attack" bonus on the table from the DMG:

\begin{array} {|c|c c c|c|} \hline &\color{gray}{Stat}&&Total&\color{purple}{Highest}\\Lvl &\color{gray}{Mod} & \color{gray}{Pro} & Mod &\color{purple}{Slot\ ^2} \\ \hline 1 & \color{gray}{+3} & \color{gray}{+2} & +5 & \color{purple}{1} \\ \hline 2 & \color{gray}{+3} & \color{gray}{+2} & +5 & \\ \hline 3 & \color{gray}{+3} & \color{gray}{+2} & +5 & \color{purple}{2} \\ \hline 4 & \color{gray}{+4} & \color{gray}{+2} & +6 & \\ \hline 5 & \color{gray}{+4} & \color{gray}{+3} & +7 & \color{purple}{3} \\ \hline 6 & \color{gray}{+4} & \color{gray}{+3} & +7 & \\ \hline 7 & \color{gray}{+4} & \color{gray}{+3} & +7 & \color{purple}{4} \\ \hline 8 & \color{gray}{+5} & \color{gray}{+3} & +8 & \\ \hline 9 & \color{gray}{+5} & \color{gray}{+4} & +9 & \color{purple}{5} \\ \hline 10 & \color{gray}{+5} & \color{gray}{+4} & +9 & \\ \hline 11 & \color{gray}{+5} & \color{gray}{+4} & +9 & \color{purple}{6} \\ \hline 12 & \color{gray}{+5} & \color{gray}{+4} & +9 & \\ \hline 13 & \color{gray}{+5} & \color{gray}{+5} & +10 & \color{purple}{7} \\ \hline 14 & \color{gray}{+5} & \color{gray}{+5} & +10 & \\ \hline 15 & \color{gray}{+5} & \color{gray}{+5} & +10 & \color{purple}{8} \\ \hline 16 & \color{gray}{+5} & \color{gray}{+5} & +10 & \\ \hline 17 & \color{gray}{+5} & \color{gray}{+6} & +11 & \color{purple}{9} \\ \hline \end{array}

^{1 Using a standard array or point buy with the upper limit capped at 15}

^{2 Compare to the spell scroll level}

The caveats:

If a character gains a magic item, it would boost their casting mod sooner than what's listed on the table (ceteris paribus).

Likewise, if a character takes a feat instead of boosting their stat, they will be "behind the curve" (ceteris paribus).