+1 to the hydrometer. It's a very valuable tool.

As far as yeast pitching rates. Here's some info that has become a general rule in my brewing. Pitch plenty of healthy yeast into a well aerated wort prior to fermentation.

Here's the math from mrmalty.com, JZ's website.

Calculate the number of yeast cells needed. For an ale, you want to pitch around 0.75 million cells of viable yeast (0.75 million for an ale, 1.5 million for a lager), for every milliliter of wort, for every degree plato.

(0.75 million) X (milliliters of wort) X (degrees Plato of the wort)

There is about 3785 milliliters in a gallon. There are about 20,000 milliliters in 5.25 gallons

A degree Plato is about 1.004 of original gravity. Just divide the OG by 4 to get Plato (e.g., 1.048 is 12 degrees Plato).

So, for a 1.048 wort pitching into 5.25 gallons you need about 180 billion cells.

(750,000) X (20,000) X (12) = 180,000,000,000

As an easy to remember rough estimate, you need about 15 billion cells for each degree Plato or about 4 billion cells for each point of OG when pitching into a little over 5 gallons of wort. If you want a quick way of doing a back of the envelope estimate, that is really close to 0.75 billion cells for each point of gravity per gallon of wort. Double that to 1.5 billion for a lager

One Wyeast smack pack or White Labs vial is equivalent to 100 Billion cells.