I like and use this method and the rounded #s TLM mentioned are easier
to remember and don't require thousands place calculations.
Where
most people have a problem with this method is over-estimating weight
by not properly placing the tape. You want to find the smallest snug
circumference from directly behind the hog's front legs. So the tape
has to be perpendicular to the body (not angled forward or rearward) and
the measurement be taken directly behind and ideally snugged into the
arm pits.
I usually remeasure 2 or 3 times, resituating the tape each time to try to get a better placement and then run with the smallest number. That usually gets me with 5% of the actual weight when I have checked it against the scale.
There have been some reports from folks who indicate that the tape method can't produce good numbers and usually report some sort of huge error that is over the actual weight. I don't doubt they had a huge error. The huge error is undoubtedly tape placement.
As noted, this isn't a perfect method, but it beats the hell out of carrying a scale around and it beats eyeballing or lifting estimates.
Does it work for small pigs? Sure, but once you get into piglet range, it does not. In fact, if you get a pig with a H/G less than 20" the pig will have a negative weight estimate.