Residual Sugar

So, I decided to try my hand at making a cider, because it seems like the harvest season is rapidly approaching.

Here's the recipe:
5g 100% apple juice
3.2# Brown Sugar
Spices
Safale S-04

OG was 1.07

Everything has gone well.
I checked the gravity the other night and it was sitting at 1.02. I know it could go further as is in evidence by the still bubbling airlock.
Now, I want my wife to enjoy this with me so I was thinking about bottling sooner rather than later and not letting it go on to 1.0 or lower.
Problem is, that at this point I have know idea how much dextrose I would add to prime. I'm afraid that the residual sugar that the yeast is still working on plus any amount of dextrose that I would guess at would lead me to a batch of bottle bombs.

Is there a formula that I can get that will help me figure this out?

I suggest pasteurizing in the bottle like you will find in the sticky thread. It will still be sweet at this point. If you want carbonation, add sugar as you would to beer and wait to pasteurize by testing a bottle every other day after the third day in bottles. Once you have the desired carbonation level, then pasteurize.