Calculating ABV; adding sugar during fermentation

[Icedpyre]

So I just did my first all grain recipe this week. I ended up having to borrow some brewing equipment, as my pot wasn't large enough to mash 4.7kg of grain. It had issues holding a constant temp, and i think the center actually got too hot, leaving me with unfermentables.
Fast forward a few days. I brewed sunday, and by late Monday i was only seeing the odd burp from my airlock. By Tuesday morning,all was quiet. I'm not sure if my yeast was bad, or if there actually wasn't enough fermentable sugars.
I had considered racking it after 2 days, pitchong another yeast package, and racking it on to 1kg of dextrose(my OG was about 20GU less than it should have been). This brings me to the main question.
If I have to add reasonable amounts of something that alters my gravity, how then will i be able to calculate my ABV when all is done?

Eg. My OG was 1.035. 4 days after brewing, i rack on to 1kg of dextrose. The grav has gone down(i hope) due to fermenting, but now is higher due to the addition of sugars. How will i calculate the end ABV?

Also, is my plan terrible? I will likely enact it tomorrow unless warned otherwise.

[sleepyjamie]

So I just did my first all grain recipe this week. I ended up having to borrow some brewing equipment, as my pot wasn't large enough to mash 4.7kg of grain. It had issues holding a constant temp, and i think the center actually got too hot, leaving me with unfermentables.
Fast forward a few days. I brewed sunday, and by late Monday i was only seeing the odd burp from my airlock. By Tuesday morning,all was quiet. I'm not sure if my yeast was bad, or if there actually wasn't enough fermentable sugars.
I had considered racking it after 2 days, pitchong another yeast package, and racking it on to 1kg of dextrose(my OG was about 20GU less than it should have been). This brings me to the main question.
If I have to add reasonable amounts of something that alters my gravity, how then will i be able to calculate my ABV when all is done?

Eg. My OG was 1.035. 4 days after brewing, i rack on to 1kg of dextrose. The grav has gone down(i hope) due to fermenting, but now is higher due to the addition of sugars. How will i calculate the end ABV?

Also, is my plan terrible? I will likely enact it tomorrow unless warned otherwise.

I'm not sure why you would rack onto a kg of sugar after you think fermentation is complete. I'm guessing the yeast won't be able to chew through that much sugar after it has finished with your original wort. Refrain from using an airlock as a method to measure yeast activity. Use your hydrometer to measure final gravity. You could use beersmith to determine ABV after adding the sugar using a hydrometer.

[Icedpyre]

right, no I'll use my hydrometer regardless. I've just never seen a fermentation occur where almost ZERO gas was vented. The only reason to considering a rack onto dextrose, was in the event that the yeast had too little fermentable sugars, to the difficulties I ran into with mashing. Don't want those poor yeasts to starve. Also, my original grain bill was supposed to hit an OG of 1.064, and it only hit 1.035. I didn't have anything to boost the gravity on hand, so I figured racking the beer into some sugar after a couple days, wouldn't hurt it.

[sleepyjamie]

I've had my airlock not bubble a few times and realized later it was a unsealed lid on the bucket. Are you using buckets or carboys?

[Icedpyre]

I do single stage buckets. I did indeed check the seal after 1 day not seeing any bubbles. Checked the airlock seal as well.