With apologies to KY, we use exclusively home made vanilla extract, and have been doing so for years.
The exact ratios depend a bit on what kind of beans you use. The most common "conventional wisdom" on the net seems to be either 3 beans to make one cup, or 6 beans to make a "fifth" (the size of most liquor bottles -- 1/5 of a gallon, and 4/5 of a quart). We generally use 12 beans per bottle (fifth) of liquor, which is darn close to 3 beans per cup. We store for at least two months weeks before using. We make our extract in the liquor bottle itself.
Here's how. Open a fresh bottle of an appropriate riotous spirit, and take a swallow or two from the bottle to make room for the beans (very important). Smile. Lay the beans out on your board, score them lengthwise, and scrape them. Put the scrapings and beans into the bottle. Replace the cap. Keep smiling. Store the bottle. Walk away. Th... th... th... that's all folks!
No need to dilute the booze -- in fact not a good idea at all. If you're going to dilute liquor, you should put some ice in it, along with an orange slice, cherry and parasol on top. Vanilla extract is not a cocktail, and an 80ish proof hooch is great as is.
When the extract has sufficiently aged to be usable, we transfer about a cup at a time to a bottle we keep in the cabinet above the baking area, while storing the rest in the bottle in which it was brewed -- along with the brewing beans (yes, the extract in the brewing bottle does get a little stronger and tastier over time). When the brewing bottle is exhausted, the beans are too tired to be reused for the purpose, so we bury them in a bowl of sugar. In a few weeks -- voila! Vanilla sugar.
The trick is to start new batch soon enough to have it ready by the time you go through the old one. You get spoiled. You won't want to go back to store bought while you wait the two months.
Unless you have a very specific destination in mind -- coffee for instance -- scotch would be a very poor choice of alcohols. Vodka, rum, brandy (especially cognac) and bourbon (if you must use whiskey) are all good. We usually have a vodka and cognac version going at any given time.
We mostly use rather inexpensive vodka (less than $8/fifth); and cognac ($10ish -- like Raynal VSOP, Christian Bros XO, etc.); Brugal rum (which is good enough and dark enough to have nuances). Whiskeys (and whiskys) have so much individual character, it pays to use something good. But a bottle of Jack or Bulleit is going to make for some expensive vanilla -- I leave it to your sound discretion.
Again, scotch really strikes me as strange. You surely wouldn't want its peat-smoke character in most cookies, would you? I don't see it, but would like some insight. Who told you scotch?
You'll find that your homemade extract, no matter how many beans you cram into the bottle or how long you store, max out at about half the strength of commercial extracts. Adjust accordingly. You'll also find that they taste significantly better than even the best commercial extracts -- so you'll also tend to push the vanilla a little harder in your recipes.
FYI, the best online vanilla e-tailer is Golden Gate, aka Vanilla Saffron Imports: Vanilla Beans Order Page
. 1/2 pound of good quality Tahitian currently runs around $16 there.
Hope this helps,