ok so let me see if i got it:
you bought an eldritch bow and you are testing it against falcon bow?, that means +1 holy ml vs +1 attack, well there are several things here to take into account
1- you are comparing 2 way different elements, while holy damage is mostly neutral to effective against something in a hunt, physical often to be neutral to ineffective.
2- you are right, you shouldn't use momentum to test since it hinder your samples, there's nothing like that to boost directly your autoattacks since onslaught works for any kind of damage not only auto attacks.
3-your testing have a major issue, you can't avoid the extra damage overall, actually you must take into account every source of damage for your overall performance cuz you will always do more holy damage with eldritch bow on top of falcon bow and divine caldera is included, while that doesn't mean you will always do more in a hunt since these are different elements.
4- magic level, even if is specialized only in holy ml, isn't a resource easily obtainable and can't be replaced while you can actually replace the extra attack damage with distance skill and in cyclopedia you have a valuable source to know exactly your attack value in the moment.
5- other factors outside of my knowledge as your divine grenade tier.
So in overall and with all that said i don't really think divine grenade and divine caldera will output more damage than +1 attack, i could be wrong but i can't test myself, yet i recommend you a different approach, since you can know with ease how much attack value cost each attack point, i recommend comparing which is easier to recover 1+ ml or 1+ attack, that were doing your task easier since for a real test to happen you must do something like this:
set a time lets say 10 minutes = 300 turns (300 autoattacks)
know the attack value lost per 1 attack = ? i will use as example -10 attack value (that's the avg damage of your AA)
so if per 1 attack point you lost 10 attack value, in average you are losing 3k damage each 10 minutes.
know the extra damage of the spell = ? i will say 50 as your example for divine grenade and 20 for divine caldera
know the rate of each spell = divine caldera is 1 each 2 turns while divine grenade stage 2 is 1 each 5* turns
so if you don't miss any divine caldera you will do 150 in 10 minutes with 20 extra damage = 3k damage and if you don't miss any divine grenade you will do 60 in 10 minutes with 50 extra damage = 3000.
in overall you will be doing 6000 vs 3000 of auto attacks (well, actually this part is wrong, from 60 iterations of 5 turns you get to use 2 divine caldera, 2 runes and 1 divine grenade so you use 60 grenade, 120 caldera and 120 runes with a total extra damage of 2.4k+3k =5.4k vs 3k) which means you were doing more damage with eldritch bow but to know this you must collect that data, any other kind of calculation is subject to bias and other factors outside of your control.