Background: the sugar in milk is lactose [1], which tastes about 16% as sweet as table sugar (sucrose). When you make lactose-free milk (Lactaid) you use the lactase enzyme to break each lactose molecule into glucose and galactose. Both of these sugars are sweeter than lactose, but still not as sweet as table sugar (74% and 60%). This is why lactose-free milk tastes mildly sweeter than regular milk.

How much sugar should we leave out of recipes to account for this? There are 12g of lactose in regular milk, which will taste about as sweet as 2g (12g*0.16) of table sugar. The lactase process doesn't remove sugar, so there are still 12g of sugars in lactose-free milk, split 50-50 between glucose and galactose. [2] This means the apparent sweetness of lactose-free milk is 8g (0.74/2*12 + 0.60/2*12) of table sugar. For each cup of milk we need to lose 6g of table sugar, which is 1.5 teaspoons.

(In rough taste testing this is about right to me, but my sister and father both thought lactaid was somewhat sweeter than 1.5t of sugar in 1c of regular milk.)

[1] My rough reading is that milk has other sugars in negligible amounts, but I haven't found an authoritative source.

[2] Technically they're only split 50-50 by number of molecules, not by grams, but since lactose has twelve carbon molecules and splits into glucose and galactose with six each, it should be very close to 50-50 by mass as well.