When I get my period, I tend to have some brownish discharge or "spotting" for two or three days, then a day of nothing, and then a proper bleed for 3 days or so. A woman is supposed to be most fertile 14 days after her last period starts, given a 28 day cycle. So... to figure out my most fertile period, would I count from the first day of discharge, or proper bleed?

I would count from the first day of discharge, as I think that indicates the shift in hormones. If you're trying to concieve, I'd try from day 12 through day 16, if you're trying not to, then I'd be careful from day 10 through day 20, keeping in mind that NO method is perfect, except total abstainance.
They now make some wonderful gadgets for monitoring fertility, available at any pharmacy off the shelf...at least here in the States.
You might also post this at HerPort.

I've read that the interval from ovulation to the onset of menses (beginning of the period) is fairly constant, while the interval from the period to the next ovulation is much more variable.

Therein lies the difficulty with the "rhythm method" of birth control.

In order for you to accurately distinguish when YOU ovulate I recommend taking your Basal body temp.
http://www.ovulation-calculator.com/basal-body-temperature.htm

this site has a chart
http://www.sisterzeus.com/bbt_risk_of_preg.html

in addition to taking you basal temperature, you can identify the time period that you are most fertile by the mucus you get when you wipe after going to the bathroom. fertile mucus is clear and vre stretchy, much like a raw egg white. less-fertile mucus is tacky and opaque creamy white or yellowish. this does not mean that you can't get pregnant still, but it is less friendly to sperm.
are you trying to get pregnant?

Thanks!
Not trying yet, but getting married on July 4th. We'd like to start trying for a family asap.

This website has some really good information about cervical mucus and monitoring fertility.

The reason that the mucus becomes clear and stretchy is because it can actually assist the sperm get to the egg more effectively. Pretty cool, huh.