How much sugar to boost ABV a few percent?

I normally make cider by buying juice and adding champagne yeast - nothign else. Just let it ferment (2 gal at a time) for 2-3 weeks (or more) then bottle, adding some more juice at bottling time to carbonate it.

I am very pleased with the outcome. I don't know how strong it is (no readings) but would guess around 5-6% from the taste/effects. Does that seem likely?

If I wanted to boost the strength of my next batch a bit, how much sugar would I add to raise it a few percent? Is there a sort of rule of thumb for estimating this? Various recipes seem to give very different amounts.

How much are you fermenting at a time? (1 gallon or 10 gallons) What type of sugar are you wanting to add?

Table sugar, corn sugar, honey and brown sugar are going to be different when weighed in "cups".

I typically add about 2 cups of compacted brown sugar per 5 gallons and it typically raises the gravity about .010 +/- points. My measurements might be different because I might not compact it as much as someone else. (or my brown sugar is a different brand)

Without gravity readings, it's hard to predict how much sugar will effect your final ABV. I would suggest adding your sugar and take gravity readings until you get your juice up to about 1.060. This will typically give you a final product in the neighborhood of 7-8%.

Most apple juices/fresh pressed ciders range from 1.030-1.050. If it had an OG of 1.050 you are at about 6.5% ACV.
If it was 1.030, you are looking at 3.9% ACV (assuming ferments to 1.000).

If adding 1 cup granulated sugar to 1 gallon you will raise SG by 0.020 or approx 2.6%, if ferments to 1.000.

Once you hit 7% you fall into wine world, under most standards, so it can take a bit longer for things to age/mesh.