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Your proficiency modifier for a spell’s attack roll is the same as your proficiency modifier with spell rolls. Spell attacks are unarmed, but they don’t apply any special benefits from your weapons or unarmed attacks, nor do they deal any damage outside of what’s listed in the spell. Melee touch attacks have the finesse trait (see page 182).

You use dex or str for melee and you use dex for ranged. The proficiency bonus you add is the one you have for spell rolls.

Something to keep in mind with assurance though it that it gives you a modified roll. It is significantly worse than take 10.

If you get it at level 1 (human or rogue) with a skill which uses your primary stat then it becomes the equivalent of 'take 5'.

Assurance is worse than 'take 10.' Assurance lets you not roll to get a modified 10 as the result.

Example. 1st level rogue with 18 dex and training in stealth. Average roll is 15.5 (10.5+5).
If the rogue uses assurance then the roll becomes 10. Not 15, but 10.