If your swatch changes size after washing and drying, I would say convert the number of inches in length specified by the pattern into number of rows.
So if it says work until 10 inches long, and they want (and you've achieved on a washed swatch) gauge of 4 rows per inch, ten inches equal 40 rows. Before blocking, your 40 rows may be smaller - say 8.6 inches - but if you are getting gauge on the swatch, after washing the knitted piece, you know the gauge will reach 4 rows per inch.
So knit those 40 rows, even if that gives you less than 10 inches.
If your gauge after washing (from the swatch) is 4 rpi, then the item (after washing) should equal 4 rpi also, over 10 inches that should equal 40 rows.
Does that make sense?
Another option I suppose you could take: If your gauge on the swatch at first is 3 rows per inch (rpi) but after washing reaches the specified gauge of 4 rpi, and it calls for ten inches of knitting, you could do 7.5 inches, because you know it will increase to ten inches. That would work, but I recommend the first option.