About half way through my foot, I check my stockinette row gauge. I use this gauge to determine when to stop my foot and start my toe. I count the rows required to knit the toe for pattern that I'm knitting. Lets say my toe has 21 rows and my row gauge is 12 stitches to the inch. This means my toe will be 21 / 12 or 1 3/4" long. I need to stop my foot 1 3/4" before the end of my foot.
To determine if my foot is long enough, I try on my sock and stretch the foot about 1/4" and measure from this point. I want a little negative ease to get a snug fit.
If you aren't sure about your toe fit, thread a lifeline before starting the toe. This way you can easily rip back and start again. (Another top down benefit, not so much to rip out to get a good fit.)