Using the sole or palm as your ‘swatch’, measure how many rows you get in either 1 or 2 inches (the latter will be more accurate).
If you used 1”, that is your row gauge: ___________ rows/inch
If you used 2”, divide by 2: ____________rows/inch
Calculate, based on the pattern, how many rows are required. Example: The toe begins with 48 sts and you need to end up with 16. The pattern calls for paired decreases every other row (4 sts decreased per decrease round). This averages to 2 decreases per round. 48-16=32. 32 rounds/2 decreases/rnd = 16 rnds of knitting.
Divide this number by your row gauge. For example, if your row gauge is 10 rows/inch, knitting those 16 rounds should give you 1.6” of knitting. Thus, you should start your toe decreases when you have reached a length 1.6” less than what you want your total length should be.
If you’re not there yet, continue knitting in stockinette til you get to that point….based, of course, on your row gauge.
This works for thumbs and the tops of mittens as well.