Calculating an item's attack bonus

[Darrell Impey UK]

If a magical item triggers a spell when it is activated, but does not use the Cast a Spell activity, how it the attack bonus calculated?

For instance, "Activate [one-action] to [three-actions] (concentrate); Frequency once per day; Effect The tattoo casts a 4th-rank blazing bolt, with the rays emitting from your eyes."

[Errenor]

It would be very nice if you named the item and provided the link.
Sun Sight is level 10, but it thankfully has counteract mod for the same action (+19), so use it.
Otherwise, they should've printed the modifier. It's an error.

[Darrell Impey UK]

Errenor wrote:

It would be very nice if you named the item and provided the link.

Sun Sight is level 10, but it thankfully has counteract mod for the same action (+19), so use it.
Otherwise, they should've printed the modifier. It's an error.

Thank you. Whilst I understand that, I chose not to post the item so that I could learn the calculation method and apply it to any other cases.

[NorrKnekten]

The answer is that it isn't calculated, Typically the item lists the modifier/DC but otherwise its a safe assumption that it follows the table for Magic Item DCs in GM Core.

[Darrell Impey UK]

And therefore an attack bonus would be 10 less than the DC?

[Errenor]

Darrell Impey UK wrote:

And therefore an attack bonus would be 10 less than the DC?

Yes, exactly.